Miniyachts can sail on a variety of surfaces, including tarmac, concrete and hard-packed gravel, so car parks, airfields and yards are potential sailing sites - but you will need the owner’s permission to sail first.
Tightly grazed sheep pasture, like the fairway of a golf course, or the outfield of a cricket pitch, can be great fun. Some Commons or Parks are suitable, and longer grass needs more wind - and lower tyre pressures.
Do seek permission first, and do remember that everyone else has as much right to be there as you do.