We were on the Jewel OTS transatlantic crossing in September.
We were in England for a week prior to the cruise. We were actually in Windsor, which is about 7 miles from Heathrow. Our TA arranged for us to use the RCI transfers, but instead of going all the way into London to Victoria coach station to get the bus, we were able to go back to Heathrow and pick up the bus there. We went to the arrival area, which is after immigration/baggage claim/customs. We had our luggage with us. There were several RCI agents waiting, and we identified ourselves, said we were going to Harwich with them. They took our luggage, gave us a colored sticker (to identify which departure time we had), and took us to our bus. The next time we saw our luggage was when it arrived at our cabin.