As DominicB says, no additional security at all. As EXP with a single guest all 3 fast tracks are available for your reentry. North, South and First Wing (the latter gets you straight to the lounge and is recommended)
If you have cabin luggage not wanting to lug around London and/or want a shower/snack before heading off, you can first go to the first lounge, baggage store there (proper manned cloak room). Shower/eat and then head to immigration enroute to London (lookup the shuttle train to T5C and back system)