Thing is, you're probably not going to get half the average in the UK -- you'd probably be lucky to get one-fourth the average. Juan in Malaga will do better, but in a climate with average or worse-than-average cloudiness, you're not going to get as much power as you might expect. Have you ever tried it?
I have a panel similar to the one you link to, connected to a small charge controller to harvest the electricity and properly feed a battery. I've been using this setup to charge a couple of lead-acid batteries that run the lights in my one-room office (two 3-watt DC LED light bulbs) for over a year now, just as an experiment. I'm in the northeastern US, which is probably similar in cloudiness to you in the UK, and I'd be thrilled to get half of the theoretical maximum rated daily power from my panel. As it is, on a sunny day in the summer I can get about 80% of the rated output, for about 4 to 6 hours. Outside of those hours I get almost nothing, partly because of the angle of the sun and partly due to shade from surrounding trees and buildings. And on a sunny day in the winter I don't even get 4 hours of usable sun, since the it's so much lower in the sky.
Shade just kills the output -- any shade at all. Clouds, or trees, or buildings, or any other source of shade. One surprising aspect of solar panels is that if you shade even a small part of a panel, the output drops dramatically -- way more than you'd expect. A panel that's rated at 50 watts may output 40+ watts in full sun here where I live, but if you put half of your hand in front of the panel, shading maybe 5% of of the panel area, your output drops to 5 or 10 watts. This is why I don't think most people in most places would get even half of the maximum rated power -- is the entire roof of your car really in full sun half of the time between sun-up and sundown, on average?
South-facing solar panels on the angled roof of a house in a very sunny and treeless area -- those can work really well. Tilted to capture maximum solar energy, no clouds, no shade from trees or other buildings -- that's just ideal. But the roof of a car in most places, not so much. Based on my own experience, I just don't think you'd harvest enough energy to be worth the expense.