Assuming you collected your car around 1:30 (offsite from T3) you could expect to arrive around 3:30, perhaps a little earlier. Avis have a return office at T5 so you would need to leave by 6 to give sensible contingency. There is a long stretch of roadworks on the M3 that really slow the journey down.

But, really, is driving 160 miles on some very busy roads really the best use of a stopover. While I find gardens utterly dull somewhere like Hampton Court is interesting and easily combined with dinner in Windsor. The BA lounge catering is nothing special so much nicer to dine out.