I would reiterate what BG says. Go for a Mares mask if your funds will run to it, A snorkel doesn't need to be too fancy, and restrictions at the top end will make it more difficult to clear.
If you buy open heel fins you WILL need neoprene boots as well, regardless of what they try to tell you (sell you). If you don't go for the open heel ones, make sure you try them on with bare feet, don't just go on the size on the fin, there are huge variations in actual size. It should be a snug fit without scrunching your toes up. If it's loose it will probably move about in the water and chafe on your feet. (Personally, I'd go for open heel and boots. I use Avanto Quatro ... best fins in the world, but for a beginner, a cheaper pair will probably suffice).
When you get your mask, again try it on. Don't buy from a website/mail order unles you are buying a model that you have tried. Offer the mask up to your face and breath in very slightly through your nose. The mask should stick on to your face and be comfortable. It if falls off it's too big, If it feels tight, it may be too small. A poorly fitting mask WILL leak. Finally, get a neoprene strap cover (slap strap) for it, it will make it a lot easier to take on and off than just the bare silicon strap. Oh yes, it MUST be tempered glass, don't get plastic lenses, Get a low volume one as well, it will make mask clearing much easier during (and after) your training.
Go to Stoney Cove and have a look at what they've got. Talk to them, tell them what you need. The staff there are really helpful and will sort you out. (That's exactly what I'm doing today ... taking some new trainees up so they can get their pool kit).
You might also want to consider doing your training before you go. You will spend two days of your holiday in the pool and your first 3 evenings doing your classroom work. At least get your theory and pool sessions done before you go so that all you need to do is your open water stuff.
Oh yes, one last it of advice ... do it with BSAC
