IIRC the main reason given for not introducing convoys was the total number of arrivals & departures from British ports compared to earlier wars and the impossibility of providing sufficient escorts. However, this figure was for ALL sailings the bulk of which were coastal Once the oceanic only sailings were extracted the figures became much more manageable.
Exactly the comment I was going to make. One of the posts above suggests that the majority of British tonnage was oceanic - this may well have been the case but if so those ships would have spent a considerable portion of their time in port or away from the British isles. The extremely large number of sailings and arrivals that apparently daunted the admiralty was made up in large part of coasters pottering back and forth carrying small cargoes from port to port .Escorting them was not practical but also not necessary.
I believe in WW2 convoying was started from day 1 - which does suggest that the negatives to convoying are not so overwhelming.