wtf! lol
If you were getting 185/55/15s then yes, £300 is about the price I'd expect from a garage as it's not a cheap size, but the 195/50/15s make sense as they only give a small difference in ride from what I've heard, mostly stiffening the ride up through reduced sidewall height.
Oh, and if you're getting 2 tyres at a time though, put the new ones on the rear!