There are at least two networks what offer unlimited data, both have good coverage, why would you need more?
Three PayGo for 15 GBP/30 days
GiffGaff (O2) PayGo for 12 GBP/30 days
The fair use is around 90 GB/month for Three, GiffGaff claims they haven't got one.
Google Music is now available in the UK too, and 20,000 files can be stored for free.
BTW, streaming doesn't use lots of data, if you combine it with offline music you'll probably don't need an unlimeted plan.
It doesn't require lots of bandwidth either, it works fine with sub-3G connections too.
But as it was mentioned before you can download music to your phone so you don't have to stream all the time.