Plenty of threads on the UKForum. Just spend a few minutes looking through
http://www.flyertalk.com/forum/u-k-ireland-484/
Agree no need for the Heathrow express - why do people suggest this as the default option? It's often not the most appropriate way,
It depends where you want to visit and the best way to get there follows from that.
If he's already checked in etc then no need to be there 2 hours before and could cut that down to an hour to allow for security and gate closure time.