In case you end up with less than 5 hours (delays, immigration queues etc) and have nothing better to do, there is a Hilton linked to the terminal which BA arrives into (a longish 10 min walk through a tube like walkway) which looked to be the best bet to spend a few enforced hours at the airport. It had a quite pleasant looking coffee shop and restaurant on the ground floor and almost certainly has a shuttle that would take you to the terminal you are leaving from.
Much more pleasant than either the BA J or F lounges and an opportunity to put yourt head outside and get some fresh air also.