This being in the UK the norm would be not to tip anything (no-one will bat an eyelid) but as an exception maybe a couple of pounds. Anything more would be overkill. Sadly (reported elsewhere on FT), in other parts of the world (e.g. Asia) cultural insensitivity through over-tipping has led to a degradation of service towards those who do not tip, despite not tipping being the norm everywhere outside of the airport. Would hate to see a move towards this slippery slope in the UK.