i've never been a fan of international sim cards so i'm going to be biased and say get a local one XD;;; if you will call your mum for a long duration, O2 have/had (don't know if it's still around) a tariff where you can get free international minutes if you top up a certain amount. there's also another one where you can pay a certain amount per month (or one month only in your case) and you can get unlimited texts/calls in UK, which you can use if you will be using that bit more.

ultimately, if you're not making a lot of calls, then £25 seems like quite a lot of money for a card with no added things; but of course that's me and you might think differently.