Just used the limo service a couple of days ago from LHR to Exeter (~ 3 hr ride). I was pondering over the same tipping question. My driver was very nice and had no problem with the long drive. I gave him ten pounds (mostly because I didn't have a five pound note on me). Seeing as it cost me twenty-one pounds to take the National Express bus back to LHR today, the tip was not really that big a deal. I also think he appreciated it very much.