Once you're in the UK and free to move around, there's no problem with moving around to departures. You don't have to go outside at T3. There is an inconspicuous path between the Caffe Nero and the lifts in the arrivals area. On arrival, go towards the landside Caffe Nero and keep going past it and you'll be in the landside checkin area.
Annotating
https://www.heathrow.com/file_source...row_T3_Map.pdf
Arrivals landside (green "11" is Caffe Nero):

to departures checkin area: