Hi Foobs!
I think there is a category for 'other' on the DVLA form. Seriously though, don't complicate it. If you have Type 1.5/LADA and it's insulin treated, it is an autoimmune form of diabetes and usually lumped in to Type 1. The DVLA will be OK if you call it Type1 - their only interest is in the insulin use. The more you complicate things with them, the more likely you are to lose your license unnecessarily. Keep it simple.
As for your diagnosis, if you haven't already had the tests to distinguish your Type, the doctor is just guessing. Ask for a GAD antibody test to see if you have an autoimmune form of diabetes and a c-peptide test to see how much insulin you are producing.
Smidge