Hey, welcome to the forum! It’s great to see new users here and especially one from the UK/EU.
You’re not wrong, metal marking stuff tends to be pretty expensive here in the states too.
I wrote up a post about how to work with metals a while back, it may be handy to read through. Unfortunately I can’t help you directly with the UK supplier question, but you might find the other info useful while you’re searching for a source. Check out #11: