I'd check your contract of employment - and "custom and practice " may imply this sort of term.

I'm in the UK too, and have never seen a contract as onerous as that. If I did, I wouldn't sign it. Every contract I've signed have set up very clearly the conditions under which the employer claims ownership, and they've always been reasonable and pretty much restated what most people would find fair. The only variation I've had have been whether or not I've had a duty to disclose and ask for a waiver for certain types of code (effectively stuff that is close to competing with my employer).

Trouble is if the contract is silent then I suspect that legal precedent will still stand.