Why not escort Atlantic convoys with British submarines?

Why not use T-class submarines to escort slow convoys and ambush German surface ships?

They were needed for offensive missions, and their presence would make it much harder for escorts to identify and attack German submarines safely.

T-class submarines had other offensive missions and were not spare convoy escorts. Once a British boat submerged among the convoy routes, surface escorts would have to distinguish it from German submarines before attacking. Keeping friendly submarines outside those areas allowed any detected boat to be treated as hostile. Destroyers offered a safer way to add torpedo power against a surface raider without undermining anti-submarine warfare.