iMessage isn't a thing in Europe?

No not really. It's technically available but nobody uses it. Everyone is on WhatsApp. Even companies. I never get messages from contacts on SMS either (so it's not going through fallback). I think it's because iOS just isn't really that widely used here. Not used enough to have critical mass for an iOS-only messaging service.

At least in the countries I deal with in Europe (Netherlands, Spain, France, Ireland). Perhaps in UK the adoption is higher because they have more money and thus iOS usage is higher. But everyone I know is on either WhatsApp or Telegram (and sometimes but very rarely Signal). Also we are much more socially disconnected from the UK since brexit.

As a bit of added info, the reason SMS is so hated here is because providers offered paid SMS services. You could sign up for e.g. daily weather reports and you got billed for it on your phone bill. It could be up to 1-2 euro per SMS. Some countries even up to 5 as far as I remember. This service was abused a LOT by scammers who just signed people up without consent and refused to remove them. The carriers did almost nothing against this because they were raking in the euros. This caused people to be very wary of SMS. Most people I know never use it anymore. They get worried when they receive something because they are afraid they'll get charged. Which can really add up if they do it from the start of your billing cycle. So its use as iMessage fallback is also pretty nonexistent.

So this is also why I am so wary of RCS and the carriers. They have played a deplorable role in the 2000s/early 2010s. Really cashing in hard with small bundles and insane out-of-bundle charges for SMS, the pay service scams etc. It was really their cash cow. So my trust in them is forever lost, I will never trust them to provide more than just transporting neutral bits from A to B.

It's also why I will never sign a contract with telcos and always use prepay. That way they can never take more of my money than I have in credit.