The proximity of Farnborough and Gatwick limits your options.
To the NE, I flew to the former Wisley airfield and followed the M25 to the East for a while. To the SW, I flew to the Devil's Punchbowl at Hindhead. You can also reach the coast at Brighton, Littlehampton or Portsmouth, but that's probably a one-way flight from Guildford.
Have a current flight map and NOTAM on hand, because it's a complex airspace all around, you must avoid big areas, or keep relatively low (below 2000 ft).