Bennies is a slang term used to describe "Benzedrine" off of the street and/or “Benadryl”. Ironically enough the drugs are very different. Benzedrine is similar to that of Methamphetamine. While Benadryl gives the user a nightmarish high with lots of cramps and visual effects.
A phrase to describe someone receiving the maximum allowance from the UK Welfare System. In the UK these are know as Social Benefits, hence being on "Full Benefits" or "Full Bennies"