For sure solar isn't as effective in the UK but we're lucky to have shallow sea nearby for offshore wind (onshore wind being opposed by the same kind of NIMBY clowns that are a true biblical plague in every country). But if solar is cheap enough you may as well deploy it even where it's not especially efficient. Then you can start to look at things like transmission lines or whatever energy source works best for your area.
Edit: it's a reasonable point that dollars spent in the 2nd cloudiest state on solar aren't particularly well spent, but that leaves you with 48(?) less cloudy states. While CA is pretty south there are 10 more states to the south of it, one of them is even called the sunshine state and is effectively an uninhabitable sinking swamp, ideal for solar!