TigerFly at Wycombe Air Park (on the M40 at High Wycombe) are the boys you want, without doubt!
They charge £80 from memory for a good jaunt off over Marlow - including aerobatics if the passenger is up for it!
They are a great bunch of guys - we've used them a few times now for friends birthdays and the like over the years... thoroughly recommended.
Their web-site is at: http://www.avnet.co.uk/tigerfly/
"Andy sang, Andy watched, Andy waited 'til his billy boiled..." (It was written for me!)