I have used the PI Terminal 5 location the last 2 times I flew from Heathrow. It is 2/3 miles from the airport, but - my experience - the bus service has been great. The bus line from the airport is Number 423 and it stops outside the hotel. The return 423 bus service goes from just across the street from the PI (not a busy road). Takes about 10 minutes. There is also the possibility to use the hotel hoppa bus (not used this.) The staff at the PI have a sheet with all the bus options and the times. Another advantage of this PI is there is a McDonalds next door (not somewhere I often eat, but can be useful:)). It does have the usual on site restaurants. The last time I went I thought their menu options had improved at bit :).
I have stayed at 2 of the other PIs at Heathrow in the past. (1) There is one actually in T4, long walkway there, but perfectly fine. You'd just have to transfer from T4 to T5. (2) I've also, in the past, stayed at the PI on Bath Road - again, its a perfectly fine PI - but you have to get across the very busy Bath Road to get there.