UK demand in winter is nearly 2x higher than summer (and will grow even more with switching gas heating to heat pumps), at the same time solar output is ~10% of that at summer. Wind energy is higher but is unpredictable and in very cold snaps (when demand is greatest) tends to result in very low wind output.

This problem is the biggest one to solve (and not sure how solvable it is). In essence it's possible to smooth out daily output with renewables, but annual output is very difficult. You are talking TWhs of storage requirement.