At last – a use for all those phishing emails you’ve been getting!

I don’t see why you can’t or shouldn’t contribute samples from outside the UK. The announcement carefully doesn’t say “you must be from UK to send data”.
Spams and scams are a global problem and AFAIK it can be a huge help to law enforcement in one country to get reports from another. It would IMO be a pity if the NCSC couldn’t do a takedown inside the UK because all the attacks had gone offshore and no one inside the UK had any evidence of incoming malicious traffic to make the case.
If you look at the average cybercrime bust these days there may be cops in dozens of countries working at the same time to effect takedowns and arrests.

This suggests that whatever mail server or service you are using doesn’t support TLS (encrypted transmission), but the phishing.gov.uk server not only supports it but understandably expects it.
What email client were you using and how did you send the email? If you use any of the popular webmail clients out there, they should all prefer and use TLS, so you might want to try sending again from, say, Outlook.com or Gmail and see what happens. (Seems to work for me from Office 365.)
If you are sending directly from your own computer and letting your email software talk directly to the other end, you can almost certainly turn TLS on somewhere in its preferences or accounts window.
Please let us know how you get along!

I think you are over-analysing this – like people who get upset when the words “computer virus” are loosely used to refer to any overtly malicious software, including non-self-replicating malware, or people who demand that “computer viruses” and “computer worms” are disjoint sets because their technical definition demands it. (FWIW, my own opinion is that worms are a proper subset of viruses, but that’s diving back into details that don’t help in common parlance.)
Yet when people ask what I do and I say “computer viruses? [knowing nod] – I work for a company that keeps all that stuff out”, they know at once what I mean. I know they know what I mean because they invariably want to ask about malware… as well as spam, and scams, and rogue emails, and people who crack encryption for good and bad, and hacking, and rootkits, and crooks who sell stolen credit cards, and the Dark Web, and SIM swapping…. and “all that stuff”.
I think (IMNSHO) that you have been far too specific by insisting that “phishing is about credentials”. I define phishing as “any electronic communication that persuades you to part with something you later wish you had kept to yourself.” That includes passwords, which are about taking over accounts, but also credit card numbers, which are about spending money, as well as addresses and other information that could later help a crook to assume your identity in some way. I think phishing also quite unexceptionably includes messages that lure you and reel you in to the point that you start trusting someone you don’t even known enough to send them money up front, or to cash “cheques” on their behalf.
I think that saying “please report phishing emails to NCSC” is an excellent and plain-English way of catching people’s attention and conveying what I think is meant – any emails that you think are out to get you in some dishonest way, well, they’d like to get a copy.
If you choose only to send in emails that link to websites with fake login pages to steal credentials, following a very strict and narrow definition of phishing, that’s absolutely fine.
If someone else chooses to interpret phishing as “a dishonest email that is fraudulently trying to lure me into a contract, an investment discussion, a money-muling job, a job application with CV included, an oil pipeline payout or even a long-term romance where I end up sending anonymous cash donations by wire transfer”, I think that’s fine too.
And I think this broader definition matches perfectly well with the NCSC’s words: “Have you spotted a suspicious email? If you have received an email which you’re not quite sure about, forward it to the Suspicious Email Reporting Service (SERS).”
Coronavirus scams, yes, included explicity. Emails phishing for banking passwords, yes, included explicitly. And every other email scam you care to think of where the crooks are the on the prowl, the email is the lure, and you are the potential victim… well, IMO, those too are included either implicitly or explicitly.
My 2p.

That is not what I said at all. You are putting words in my mouth. What I said is that it’s “not always that simple” to suppress spam by finding parts of the network to cut off.
Indeed, determining a proportionate response when dealing with spam senders is not trivial, because there is often no easy way to take down just the affected computer of the affected user without harming others. Having said that, users who get infected and therefore contribute to the spam problem may very well end in trouble with their ISP. In some countries, ISPs will kick users out if they are persistent offenders, even though they haven’t done anything illegal themselves. And spam filtering blocklists are usually fairly aggressive in throttling email traffic from devices that are busy spamming.
As an example, if you received spam from one gmail DOT com address, you wouldn’t immediately blocklist all of Gmail. Or if you did, good luck with that. In fact, even if you could surgically block a user’s Gmail account after seeing some spam, you might still be wasting your time if the sending of the email was down to a specific *device* on someone’s network, not a specific *email service*. (In other words, the spamming might depend on a specific infected computer, not a specific compromised email account.)
Likewise, if you notice malicious traffic from one IP number, you might decide to block or not block that IP number depending on how many computers you thought were on its network, and what community it was serving. If it were a free-standing suburban house, blocking it would probably feel justified, but if it were an entire high-rise apartment block, it would not. If it were an internet cafe that was a regular spam-sending offender, blocking it would probably feel like the right thing to do; if it were a hospital, it would not.
The IP blocklisting problem is compounded by the fact that many ISPs don’t give customers the same IP number every time they connect, to increase the quantity of IP numbers (32-bit IP numbers are a scarce resource) that are still available for allocation at any time.
(As an analogy – would you expect your bank to cancel all your direct debits unilaterally and close all your family’s accounts if it noticed a single fraudulent transaction on your credit card? Or would you expect a more nuanced response than to throw out the baby with the bathwater?)