Taylor Hanson in Crowthorne have an 8 seater Merc "Van" we've used before for airport runs. Normally reasonably priced. Think it was £48 for a drop-off (5am run), £58 for a pick up (midday) last time we used them.
Ascot cars are my preferred airport choice, they do 7 seaters - but worth a call to see if it's a suitable vehicle for you. Always smartly turned out drivers in clean, decent cars in my experience.