Hopefully this will gravitate its way to the top of a Google search somewhere as it’s potentially quite useful.
I managed to get hold of someone at Student Loans and they confirmed that it’s also possible to fax details back to them. The number (indeed the option to fax them) isn’t mentioned anywhere on the letter they send out. The fax number I received isn’t on their web page (that I can find) although there are numbers listed which don’t match it.
So, in case it’s of any use, the fax number for sending deferment details back to the Student Loans Company is: 0141 810 1844. Obviously you’ll need the +44 or 0044 country code in front if you’re outside of the UK.