Until strike days are announced I personally wouldn't be making any changes to my bookings at the moment especially if it involved having tp pay any fare differences. Afterall you could be moving from a non strike day to a strike day,
Personally with an early on the day TATL i would fly down to LHR the night before and stay in a hotel. Even though your plans aren't final I'd look to book a refundable rate room to lock a 'reasonable' price in..