Re: "I wonder, given Must has cracked solar roof tiles, if some variant of them would not be strong enough to sustain some light vans parked upon it, allowing the car park to become a large solar panel. Also, do the vehicles have to be charged where they operate? RM is not short of land, so could disperse some of the vehicles for charging in other nearby locations."
This is, theoretically a good idea. However, I felt the same about Solar Roadways until I actually tried to research them
There are a couple of problems.
First, they need a substantial amount of clear weather. Solar panels don't need the weather to be hot, but they do need sunny days, and work better the more sunlight they are exposed to. This is why standard panels in solar farms can move. I know that standard domestic solar panels are fixed, but the average house doesn't, relatively speaking, need a lot of electricity. It sounds like the Post Office need as much power as possible, so ideally the panels would need to move.
Second is maintenance. The cells in the panels need to be relatively clean. Easy to achieve if the panel is stuck on a roof, but a lot more difficult on a surface cars drive on, and people walk on, which is likely to end up scuffed and covered in mud, skid marks and all sorts.
Third is the cars. The idea of a car park requires it is going to have cars parked on it. If they are parked on it in daylight hours (as is likely with the post office, who tend to do most of their inter area deliveries overnight) , they are going to be blocking the light from the panels..
I know you came up with the idea of using unused car parks for these panels, but, tbh, I doubt the post office has many unused car parks in the London area, and even if they do, they are probably looking at selling the land for development, or using it to expand their offices.