Hi Happy,
I'm a languages teacher (mainly French, but some Italian).
In terms of what emy was saying, it doesn't really matter whether the school you send your DD to teaches Italian or not (most don't), they should still let her enter for GCSE as a native speaker when they think she's ready. She will need to be prepared for it, but any good MFL teacher could do that without being able to speak much, if any, Italian as the specs for most mainstream European languages are the same. I've had to coach kids for Polish, which I can't speak 1 word of, and they coped ok!
The main issue won't be her language ability, it will be her ability to think in the way examiners want; particularly for the speaking exam this is not an especially natural way to use the language so even native speakers need help e.g. sit practice papers and go through them.
By far the easiest way to go would be to send her to an English school and let her learn whatever foreign languages are offered there (French, German etc.). By the end of Year 10, she should have done enough exam technique to know what's expected and then be able to sit the Italian GCSE with a little help from teachers.
BTW, sometimes being a minority language is helpful. The Italian Embassy are really keen for anyone to learn Italian, so they dole out all kinds of freebies to any school who teaches it, sponsor prizes at prizegiving, send diplomats round to talk to kids etc.
The French and German embassies do sod all very little of this kind (at least, that's ever been seen in the schools I've taught in).