I'm thinking it's the pool frog. Your picture fits the description of them for the most part. The only thing I can't find is the white stripe down its back. Do any of the other frogs in the pond have a white stripe?
The types of frogs in the England are common frogs, common toads, natterjack toads, and pool frogs.
The picture isn't of a toad, so it's either the common frog or the pool frog. And it doesn't look much like a common frog.
Here's a pic of the pool frog.
More info on the pool frog.
And here's where I got most of the information.
Edit: Nice pic, btw.