First of all - forget the naysayers. The proposed trip is 6 months away, so plenty of time to get yourself up to speed for it.
Exeter, from a GA perspective, is dead easy. You can either prepay your parking and landing on the Aviation South West website (
Aviation South West - THE Exeter flying training school handling GA Aircraft on behalf of Exeter Airport) or in person in their offices - I'd recommend the former to save time. Booking out - file a flight plan beforehand, another 2 minutes saved. Security - get an access card from Aviation South West before you leave, give it to the man at the security gate, you're only a short walk from the aircraft.
Route: Exeter - Thruxton - Wycombe - Elstree. 131nm, so call it 1H30 Brakes on to Brakes off.
The secret of all trips like this.1) Whatever you can do before you leave the aircraft, you do. Fuelling, admin etc. That gives you time when you need it later, and you even have 30 minutes "wriggle room" at the end of the day. 2) With the plan in place, know that it will work, don't let any small problems stress you out - because they are what will cost you time. Less haste, more speed.