Not Devon or Cornwall, but take a look at
www.inishcoo.com - a big, very comfortable, house on an island with 6 double bedrooms, one single and a dormitory which sleeps 8 kids. Amazing beaches, boat trips included and swallows and amazons style holiday. I can give more details if required. There are flights via Dublin to Donegal airport which is about 10 minutes away from the port you would be collected from. It works out quicker than driving to Cornwall if you get the connection in Dublin to work well. There are 3 small cottages on the island which are rarely used so usually you get the 116 acres, over 5 beaches, fresh water lake etc. just for you.