Whilst there are 'certain' movements that aren't shouted about and notwithstanding the normal Chinook/Thames passages, any low(er) flying flights have been advertised in a generalized way so as to
not alarm the good citizens below and keep them onside. That the OP thought they heard 'several' low-flying aircraft close together the choices are few. Yes the night-time practice of the Reds does sound silly (perhaps with NVG's ?), but unless it was an escorted civilian airliner coming into LHR then what ? Any SF flying would be countered by the chance of something going wrong and carnage on the ground which would be a disaster in all senses.
By the by, only this lunchtime I looked up to see a west-bound contrail in the blue sky and a thick white 'cross' caught my eye. It was moving so slowly that I thought it must be a large party balloon, however, it then proceed to speed up and commence a large sweeping turn. Silent, unusual looking and had me scratching my head as to what it was, perhaps a drone ?
SHJ