vintage hardened - all of the unhardened squares are temporary junk that will get scratched and dinged by anything, and in combination types, slide roughly, get abraded by foreign dirt and deform easily.
Find a combination square with a hardened *head*. The rules are often hardened, almost always. but the heads wear and are sloppy. A hardened head will operate like ice and not wear out, and a used version will generally still be more accurate than a square with an unhardened head, and it will stay that way.
I don't see any right now on ebay.co.uk right now other than those coming from the US. I have two lufkins and a starrett (used tools are common here) and have had a gaggle of other used squares (including starrett, etc,) where the head wasn't hardened, as well as "PEC". the two older hardened squares are more accurate than any of the unhardened, far more accurate and far better working than the new PEC.
As far as the engineer's squares go, get yourself one that is square and checked against a combination square if you want little engineers square, but don't be surprised to find how accurate they are like gambling. an $8 square from india may be dead on, and then some cheesy anodized aluminum version like "incra guaranteed" or whatever they call them will come in a nice box and potentially be far off.
Friend of mine here got one of the latter, certificate and all. He never seemed to have luck building things square with it so he shelved it, almost unused. He bought a certified starrett square (for enormous money) and the incra square was nowhere close. $100. He bought it because he wanted a large engineer's style square but didn't want to buy an engineer's square because of the cost. I had a three square small set from india that was far more accurate and I think at the time, it was $25 for all three.
bottom line, if it slides, hardened head used is far better than unhardened new. Anything else, have one square that's true and then check the rest with it and leave it in its case otherwise so that it remains true. If you have a friend with a true square, then you don't need to spend the money on "the true one", just check a cheap square and when you find one that's true, that becomes your true square that you set aside.
