Bags can be left at the Excess Baggage Company, adjacent to Giraffe Cafe in the arrivals hall at the north end.

Windsor is a good option to kill a few hours, if you google a taxi firm they will watch your flight number and meet you in arrivals at a suitable time to take you to Windsor.

The castle is good and Eton is a nice stroll too. There are lots of nice places to eat and a couple with open terraces on the Thames (weather permitting).

On return to 5 simply collect your bags, go up in the lift and clear security before heading to a lounge....