I frequently use the A10, an excellent service (albeit a bit dodgy in the mornings from Uxbridge), at 80p with an oyster card it's the best way to do it.
It's just that on a sunny evening, finishing work at 5.30pm, with an 8pm flight to somewhere exotic like Stavanger, a nice leg stretch would be useful before being cooped up for 2 hours.
GwynM