Final update of what actually happened in LGW:

We landed in Gatwick yesterday (arriving from MCO) around 06.30 am and made our way to the flight connections. Our luggage had already been through checked to MRU.
At the entrance of the flight connections gate, the security officer examined our boarding pass and said "its too early to let you in as your flight leaves only at 21.00 hrs".
I told him that we were both BA Gold members and were going to BA First lounge to rest. He let us in without any hassle. After a quick security check of our hand luggage (scan), we went to BA lounge, had shower and a good breakfast. It was already 9.00 am by then.

We then went to BA Guest Services counter within the BA's lounge itself (at the entrance of the lounge) and told the desk officer that we wanted to go out of the airport and spend some time in London since we had a long layover. The lady happily arranged a security escort for us which took some 15 minutes to arrive, and led us all the way through to the immigration counter (through express lanes) where we were "decontrolled", and left the airport without any problem.

One important thing that needs mentioning is that when the security escort arrived in the BA lounge to take us to the immigration, he specifically mentioned that our baggage (which had already been through checked in MCO on to our final destination) were being "technically removed from the system" and that they would not be placed on our flight to the final destination unless, on our re-entering the airport, we reported back to the same Guest Services in BA lounge to confirm that we were actually back to enable BA to give clearance to their luggage handling system to load our luggage on our flight. I was given to believe that this step is necessary for security reasons.

After the quick immigration clearance (we did not need to fill in any landing cards), we went to the train ticket counter, bought two one-day off peak (after 9.30 am) travel cards for £15 each (valid for return travel from Gatwick to all zones in London), spent the day at several places in London including Kingston and reported back to BA lounge around 7.00 pm and were assured us that our bags would be on our plane. We reached our destination and our bags were amongst the first to come out.

A good overall experience, probably helped in large part by our BA elite status.

cheers