Wanted to thank everyone for your help in planning our short layover visit to Heathrow. it was definitely worth our while to visit Windsor and much nicer than staying at the airport (which we ended up doing on our return through Heathrow).
Our flight from DC was scheduled to arrive at Heathrow at 10:25 am and our next flight was scheduled for 5:50 pm--so we had about 7 hours to visit Windsor and get back in time to go through security and board our plane. With your suggestions, we booked Windsor Cars to pick us up at arrivals, and meet us in downtown Windsor at 3 pm to return us to the airport. This was more than enough time to have lunch in Windsor, walk around some, buy some treats, and make our flight.
Turns out that our flight was early and so used our global phone to call Windsor Cars to let them know that we had already arrived. We had checked our bags all the way through to Johannesburg but requested that Windsor Cars hold our carryons (minus the passports and valuables) so we didn't have to lug them around our short visit to Windsor.
The quoted price was 26 pounds cash for a normal car up to 4 people including a meet and great at arrivals, and 16 pounds return. The service overall was very good, but, the first driver did charge us more than what they quoted but, I didn't have my email printout and we were too tired to argue the point (said that the original quoted price did not include meet and greet). They didn't charge after all for holding the carryons. We ended up having a different driver on the way back to Heathrow.
Lunch was at Cornucopia where I discovered a new dessert--Eton Mess. Think strawberries, whipped cream, meringue. Surprisingly good but sweet.
We decided that we wanted a more leisurely lunch so didn't try to see the castle (other than the outside) and instead walked along the river and saw the swans, and visited the grocery store in downtown Windsor where I picked up two gluten-free packaged baked goods to take with us to Botswana. What a treat they were later on our trip! Lemon cake and bar cookies with afternoon tea.