I'm in England and also see this contract clause a lot. Personally I've always assumed that it would be unenforceable if it was work unrelated to the business, but I've not yet had reason to hire a solicitor to find out.

I'd suggest you ask a solicitor rather than an internet though.

p.s. To everyone saying "It depends if it's in the contract" that's not very helpful, as it likely is in the contract but that doesn't make it necessarily enforceable. Plenty of contract terms aren't enforceable when you actually get a lawyer to look at them. In UK contract law, contracts cannot have unfair clauses.