Cambridge UOTC recruits from UEA Norwich, just over 60 miles away and an hour and a half by coach. I believe ULOTC recruits not only from London but as far as UKC (Canterbury, Kent). Thats as far as my knowledge goes.
Basically if you want to join your closest unit I'm sure most OTCs won't mind you having to hop on a train for training weekends. You can even claim the cost of your ticket. Just don't push for the cost of the taxi from the station to the TAC.
MSUOTC only recruits from manchester man-met, and salford.....don't see any reason why some1 from bolton shouldnt get involved at msuotc if they're willing to make the journey, contact the unit and ask. Tho beware, people get ripped from salford uni, dunno what would happen if you got found out your from bolton.
But seriously, give them a ring, no harm in asking...