I have used Ray's for airport transfers, and I have used Just Airports. In both cases, I booked them about a month in advance.
This time, with our trip at the end of October, I planned to reserve with a service in mid-September...
BUT... now I am thinking.... If folks are shying away from the tube, that means more people are going to use a transfer service. Do you think they might get 'booked up' and, do you think that the rates might go up with extra demand?
I guess there is no reason NOT to book, now,.. I just wouldn't want to do it so far in advance that they 'lose' me.