FTMO and legalities



I'm new here and hoping someone can help me out.
I've been consistently profitable for quite a period of time now, returning quite a large percent of my trading account on a monthly basis (I don't want to go into details, nor is it really relevant).

I am looking to utilise prop firms such as FTMO to leverage my returns and then once my account is large enough I'll look at giving up their accounts as I'd rather have full control over how I trade and not be bothered about adhering to their ruleset.

Anyway, does anyone know the legalities behind trading an account for them? I live in the UK.
From what I understand they try to get around it by only giving us a demo account and they then us a trade copier to place the trades on their live accounts. They can stop this whenever they like.

I've had a couple of people express their concerns over the legalities of this and I just wanted to see if someone had any advice for me before I jump in and get funded through them. The last thing I want to do is get in trouble with the law over it.

Any help is greatly appreciated.