If you’re serious about surfing in Cornwall then you need multiple wetsuits. In the summer, on a warm day a shortie will be fine, on very rare occasions and just for a couple of hours boardies.
Most of the time you’ll need a full suit though, in the summer a 3/2 long or short arm will be perfect. Suits are now so flexible that wearing a 3/2 isn’t really an issue, in fact it helps to keep your muscles warm.
In the autumn and winter though things have to get thicker and depending how hardy you are depends on how thick that gets. For some winter is just a 5/3 mm wetsuit but others you have to go the full hooded 6mm!.