I always thought 'travelling' days (i.e. day of arrival/departure) were discounted (not counted), hence I doubt there would be impact on flight crew to the extent you suggest.
Having said that, would flying over (through) UK airspace count as 'working in the UK' - oops, did I just give someone an idea!
On the assumption my first statement is incorrect, would you not then be working 'in the UK' from FIR crossing to 'on chocks' - say ~45mins and from off chocks to the FIR (again ~45mins) so 45 mins per 'day' (assuming your long haul and have a nights rest!) ?
Hence you would have to do that 6 times (each) to meet 'one working day' (8-9 hours) and thus have to fly into and out of the UK 60 times a year - wouldn't you?