McLaren don't usually let anyone but somebody from McLaren drive the old McLarens.
Presuming that it is a genuine McLaren, and McLaren owned rather than privately and they aren't happy for it to be driven at 40mph by anyone but one of their drivers, they'd send one of their young kids down there. So presuming those provisos are met, then whoever McLaren have that isn't involved in F1/GP2/GP3/WsBR/Auto GP testing is a safe bet.
*edit* as above it's privately owned so it'll be an instructor.
Edited by jamiegc, 06 March 2012 - 11:19.