Not been to IOM yet but looking forward to going one day.

North Wales.. in fact all of Wales is full of really good camp sites. I can recommend
Black Rock Sands (Porthmadoc)
Shell Island http://www.shellisland.co.uk/
and all of Anglesey
Inland there is a great Forestry commission camp site at Beddgelert
Beddgelert, Gwynedd, Wales, LL55 4UU LL55 4UU‎
01766 890288
Which has free hot showers and washing and drying facilities as well!

Enjoy