That might be the right amount. No existing means of generating electricity is perfect. With present technology the best bet is a mix. I would have though the right amount of fission would be somewhat more than 25%, but maybe the UK could install a truly massive overcapacity in wind and solar if we blanket the place. Or maybe we could remain dependent on France to prop us up.
The issue with renewables is, obviously, that they're wildly variable and completely uncontrollable. It's impossible to run a modern civilisation on that basis. In theory it could be offset by having a massive excess of nameplate generating capacity (because renewables never generate their alleged output, rarely get anywhere near it and often generate little or nothing at all) and massive electricity storage. Which could work if electricity storage exists on the required scale. But it doesn't. May as well decide to power the country using dilithium crystals and a warp drive.
So we need enough controllable generation and right now that's fossil fuels or nuclear fission. But only enough. Those methods have problems of their own.
My gripe is with the 2050 target. It could be done faster. It should be done faster. At that rate they'll probably be up and running about when the first fusion power stations are being built. If that is the case, we'd be shutting down the fission power stations not long after we powered them up.
Yes, that. Also, radioactive waste is a problem. Nowhere near as much of a problem as it's often made out to be, but the less of it the better. Using a thorium cycle fission reactor would work around the issue of sourcing uranium, but that's not a well tested technology.