By co-incidence I was having lunch with a group of doctors yesterday and we were discussing medicine and sailing. (One of the doctors sails so has some idea of what is appropriate etc)
Regarding drugs, he suggested that the cheapest way of getting those that you would normally get only on prescription is to get a letter from your doctor authorising you to buy them from your chemist. For instance if you are going offshore and wish to take some penicillin whith you, it is very cheap to buy - compared with paying a UK prescription charge.
Don't know about dressings and other standard stuff - I get mine locally.
SP services supply friends who are paramedics, we put together our kit mainly from them + Antibiotics with th ehlep of a GP/sailor.
get a catalogue or go online, its far cheaper than buying one of the made up marine kits.
I was advised to equip with the very best burns kit, sterile gel pads etc, as burns are quite common, whilst cooking or working in confined spaces on engines, this is more than the MCA ask for.
web site is www.999supplies.com