I'm a very warm sleeper too, wearing similar to you year round - including camping. When on my own I use one of
these, the Travelpak lite. (can't vouch for the shop, just googled it). A very nice bag, with an insect net built in. Minimal insulation, really it's just light padding.
It rolls up small enough to fit inside my nelson longflap for minimal touring, though I tend to put it into a waterproof sack on top. I've bivvied in it (putting it inside a cheap breathable bivvy bag), and used it most of the year round (though not below zero - yet!).
It's a fiver more than your £25 limit (inc. postage), but worth every penny.