Thats a good question skaterboi. A pilot on LH reserve is allowed 2 hours to reach the car park, that would seem a reasonable start point. There may be a notice from BA soon clarifying their expectations.
It must be very hard to determine rules because we are all different. Many believe that being a passenger on a shorthaul flight to LHR is a heck of a lot less stressful than driving on the M25, fighting to find a car park space within BA’s creaking parking system and then taking a bus to the report centre.
This has been a “grey” area, allowing each pilot to determine whether his/her journey allowed them to report for work sufficiently rested to able to fulfill their legal obligations. That is similar to ensuring adequate rest is taken during the day before a night flight (no matter what pressures might exist at home), or determining whether fit enough to operate, or whether fatigued.
It would seem that BA now feel required to be much more specific about journey times to work.