There is often excess capacity in the generating system, at little extra cost, and this will become even more prevalent as wind generation expands further. You will often hear the nay-sayers repeat 'They have to turn the windmills off, if it's too windy" - this is because there is no spare demand to be filled, you cannot shove energy into the system if there is nothing there to use it. About all the storage that is there now is Turlough Hill, once that is full all you have is the current 'live demand'. There will, eventually, be some battery storage coming on-line and you can export limited amounts, assuming that the export destinations have the demand for it at the time. Fizzing up hydrogen with the nightly excess generation would just really be another form of storage. Even the old 'boiler stations' need to be kept running overnight, you can't just start them up every morning. The gas turbines are a little more 'on demand', but you still have to run some a bit light, to have spare capacity available now, if there are any failures around the system.
Having a mix of systems does allow you to defer to the best one at the time, but there is a bit of extra cost in duplication. The gamble is whether that extra cost is better or worse than being caught out by a dominant system failing, for whatever reason, technical, political or economic.