Can I get to Cheltenham from Heathrow Airport on a Saturday? If so, should I book the bus trip ahead of time?
Direct service each 2hr 30 min, no need to travel into London.
I don't think booking is 100% needed, might choose depending on scheduled arrival time vs departure. Can 'protect' your ticket with £5 additional fee. Scroll down to 'Flexible add-on'. There are several stops in Cheltenham, do you know your exact destination?
Alternatives include Rail-air bus to Reading or bus to Swindon, and then train to Cheltenham, I think direct bus preferable.
http://www.travelinesoutheast.org.uk ... for general planning, also shows Megabus service.
If getting the coach I'd always book a seat. Allow time from your arrival to get from one of the various terminals to the central coach station.
<<If getting the coach I'd always book a seat>> I don't think you can book a seat - only buy a ticket. The difficulty with this journey is the long interval between departures, if flight arrival is scheduled an hour before a departure do you buy a ticket for that service, or the next which would be 3hrs 30 min away?
It also isn't clear on National Express web-site what policy applies to overbooking, don't recall any TA posts on the subject. So as originally said I'd make a decision based on timings.
Anyway OP doesn't seemed thrilled by response.