The problem with solar power is two-fold, the lack of storage between sunny days and the lack of available sites to locate solar farms.
The French, I think, are reported to have piloted a trial of surfacing roads with solar panel material in a sufficiently skid-proof and toughened substrate. We could see where that might take us.
By my calculations, this nation averages from North to South sufficient sunlight to bring in 2.3 kWh/day/sq.m across the entire year, with practically none of that during the least sunny 4 months. If the summer excess is fed as generated methane into the old North Sea gas fields and then extracted during the winter to regenerate electricity then the storage problem is sorted. If we allocate 10% for wastage in that cycle we can rely on the remainder for the National Grid.
So, to match the current need for 400 TWh/year, I think we might have to resurface a mere 10% (25,000km) of the national roads, choosing from those which are rural and with a usable minimum 30m width across all carriageways. No land at all need be set aside, it's already allocated, owned by the nation and in need of constant resurfacing anyway. The National Grid is always close enough to the major transport arteries to transport the harvested power to storage units.
One would imagine that using this form of solar paneling in such volume would reduce the cost to an affordable rate and no doubt save on all that nasty tar demand Mr Macadam bequeathed to us.