I've worked in a half-dozen companies here in the US and never was asked to sign something remotely like that. Obviously there were IP clauses but they were all reasonable. That sucks if it's commonplace in the UK.
Not sure if it's commonplace but I've worked at a comparable number of companies full-time and there's almost always a clause that mentions it. I don't think I've ever been threatened with it being enforced, but it's one of the reasons I went independent.
