Properly laid out letter with all addresses and dates etc
Dear (whoever)
I am very grateful for your offer of a place on X course starting this September, following my gaining sufficient points at A-level. Regrettably a change in my family circumstances will prevent me from attending and giving my full attention to this course in the forthcoming academic year. I would like to ask if my place can be deferred until the start of academic year 2015-16. Please accept my apologies for any inconvenience this may cause.
Yours sincerely
signature in nice ink