Just did this... this is my recc... land at t3, and exit the airport, take heathrow express to paddington station and then store luggage at paddington. It is easier, since then, on the way back you can go direct to t5. There is a "left luggage" at paddington... after getting to paddington, get a day pass for the tube and then roam freely. I recc going to the stop of westminster for photo-ops of the abbey and big ben. Then, proceed to the tower of london... Lastly, do a little shopping at knightsbridge and then head back. That is a pretty good day in london. Make sure you have a nice companion to share the time with.