This kind of thing is totally unenforceable in the UK. The corollary of clauses about side projects is that you are unable to work anywhere else whilst you are

This kind of thing is totally unenforceable in the UK. The corollary of clauses about side projects is that you are unable to work anywhere else whilst you are employed at the company, again totally unenforceable in the UK (the case law against restraint of trade goes back to the 17th century in the UK). The only exception to this is if the work you are doing outside the contract conflicts/overlaps with the work you are doing within the contract. In that case, if the employer inserted a restraint of trade clause into the contract, that would be enforceable (that is a standard non-compete, which is perfectly legal and sensible). It is all total nonsense though. UK employers, and lawyers, are a bit mad about this kind of thing. They will insist on things like probationary periods or working "at will" for a period of time...and then they will demand that you yield 24 hours of your time whether awake or asleep to them...whilst they pay you for 7 hours. It is what happens when underemployment is high.