.UK is a weird TLD. For example, transfers between registrars are free (you don’t have to extend the domain by a year are transferring), and there’s a “push” transfer system instead of a “pull” system. You also have the ability to do certain management of the domain through Nominet instead of your registrar. Unfortunately, none of this seems to work very well and it took me months to successfully transfer my .UK, I even tried using Nominet’s paid transfer option to force the issue and it still failed over and over (although I always got a refund). Eventually it just worked for no apparent reason.

.win is fine. It might be on some extremely aggressive spam filters but it’s unlikely to affect you much.

.uk should be absolutely fine. Nominet (the UK registrar) has been around for decades, and is a member-owned not-for-profit company, which is probably why the domain prices are comparatively low.

But whilst there aren’t particularly strict rules on UK domains, using a .uk domain name if you have no connection to the UK would be a bit weird - much the same for any other country’s TLD.

You’ll probably want to avoid Russia (.ru) for obvious reasons.

In terms of the shiny new-fangled TLDs like .win, .xyz, etc. etc. - they are prone to being caught by spam filters. That’s not to say they’re going to be always or automatically blocked, simply that they will potentially get a higher spam score than ‘traditional’ TLDs. Note that this seems to apply to even some domains which have been around for a long time, but are still not ‘mainstream’ (things like .biz, for example).

Of course, all of the above might be irrelevant if you’re not planning on using the domain for outgoing mail.