At best I'd expect you would get a ruling that they should restore your free service. But enforcing it would be another matter... I expect to see Google Play Music retired (in favour of YouTube music) first :)

In most US and Canadian jurisdictions I'm aware of, small claims court can only award money damages, not injunctions or other court orders. I'd guess it's the same in the UK but don't know. So they could probably award damages in the amount of the grandfathered discount from list price for the remaining expected lifetime of Google Play Music, which would be a very enforceable award (at least after any permitted appeals) since Google is too widely present to dodge paying judgments.

Small claims court (which also exists in the US) is almost exactly the same as arbitration, but run through the government court system.

And a Toyota Prius is almost exactly the same as a space shuttle, except it's not a spacecraft.