Well I would have done exactly the same as you did,in fact did it just the other day with 2 tankers.Waited till the convoy had sailed on then finished off with torps as weather too rough for deckgun
Cant say I know what they did for real but from what I gather first thought was to save themselves esp if the hit ship was a tanker or an ammo ship as they didnt want to be anywhere near it if it went up
As for the escorts I am not sure of their standing orders
Well had a search but cant find any standing orders
This site is a good convoy site
http://www.convoyweb.org.uk/ but couldnt find anything on there or the RN site