Been quoted £100 for it, was just wondering if there were any downsides - assuming they don't cause any damage to the panel cutting etc.

One snag with most vinyl is that it contains plasticiser, which tends to escape slowly over the years causing film to shrink gradually. It becomes noticeable with floor tiles, and I would guess it may happen a bit faster with black film on a hot tin roof. But if you don't expect to keep the car for a long time that may not matter - and you can always remove it and reapply, if it hasn't become too brittle.