Transmission losses are significant

High-voltage direct current has losses of less than 3% per 1000km according to the Wikipedia article. That's not China-to-US stuff but definitely low enough to move energy around in Europe.

And they do. Up to 3-4GW of the power (maybe 10% of typical power demand) in the UK is imported from its neighbours - some Ireland and the Netherlands, lots of France - at the moment.

The French have a LOT of Nuclear power, and idling a nuclear power station, though possible, is economic insanity, so it makes sense to sell it at a discount to the British.

It's worth noting that this is a two-way street; GB sometimes exports electricity, particularly to Ireland when there's not much wind, but also sometimes to France.

Losses have to exceed 65% (i.e. 350 watts at the socket from 1,000 watts at source) for that to make worldwide solar load balancing a bad idea with current technology.