We have a
rule here in the UK, launch failure on aerotow below 300ft a landing should
be made ahead, or slightly to one side. No attempt should be made to turn
back below this height.

As a UK FI(S) and FIC, previously a Full Cat instructor, this is news to me. Where is it expressed?

I thought the rule was to exercise judgement. I always address 'where would you go now' issues with students and on check flights. In most conditions and situations I would turn back from lower than 300 foot.