Graeme,
There aren't many withing 2.5 hours of Southampton and Portsmouth - I know as I live between Fareham and Southampton.
If you want to restrict it to that, there should also be this one
Saturday 26 June – Castle Coombe
but otherwise the other nearest (I use the term losely!!) are Curborough near Litchfield, MIRA near Nuneaton and Llandow,other side of Cardiff - all of which I can do in about 3 hrs - but that is with the trailer.
For all the others I tend to stop over in B&Bs etc - although a number camp . They tell me it is good fun.......
Presumably you are on the Isle of Wight - so I suppose your travelling is dependant on ferry times etc as well. Be happy to help if possible.
Mike