In my opinion you should either rent or buy new (ideally last seasons):
Renting: Boots are new that season, you'll get a good idea what fits you, opportunity to swap if they are the wrong size/shape for you. You'll soon get to know what brand suit you.
Buy new over here: You'll be able to pick up last years boots as little as £60, I agreed it is nice to have your own boots, and comfy boots make a joyful boarding experience.
I wouldn't bother buying second hand, most boots shape to your foot pretty quickly certainly within a week. (and get pretty sweaty too, mmm)
Get yourself up to Robin Hoods
http://www.roho.co.uk/snowboard/snowboarding_offers.html for more snowboarding gear than you can shake a stick at, much of which is discounted.
Personally I've always rated Salomon boards and boots, but also like Burton stuff.
Be warned, don't be tempted to save a few quid and buying a second hand pair that don't fit. I know more than one person who has had a holiday ruined by poor fitting boots (and being too cheap to invest/hire in different ones!)
When and where are you going Dave?