Hi, not sure if you've been or are going soon but I've just come back from Samui and did my diving with a company called Coral Grand Divers who have shops all around the island and a website of the same name.
They cater for beginners and experienced divers.
They have an incredibly fast speedboat which is based at Mae Nam on the North of the island, this leaves at around 8 am for Ko Tao predominantly, but other destinations are available I believe. The trip to Ko Tao takes around 1.5 hrs.
I did a daytrip, as I have my certification, which consisted of taxi pick-up, all equipment hire (very good condition), 2 dives with an experienced divemaster, lunch at their resort on Ko Tao and taxi back to the hotel for 3500 baht.
London-Phil, if you haven't qualified i believe they can take you through this also, but I'd recommend doing your training in this country so that you can have your holiday free to enjoy the diving rather than concentrate on your bouyancy and air usage.
Or you should be able to split it and do your pool and theory stuff here and your open water dives in the resort.
All the staff were very good with european and american divemasters and Thai crew.
Whoever you go with have a great time.