Hi Lynsey,
I'm just across the water from you, and Liverpool has the same criteria as Wirral so yes, you can get surgery on the NHS. You might have to go through lifestyle and weight management first, but your BMI meets the criteria locally. The first step is to approach your GP. They should have a referral flow chart, so you won't get a referral to a surgeon straight away. You may have to try drugs (like xenical) first if you haven't already (6 months each, nurse monitored was Wirral's requirements).
It isn't fast round here, so be prepared to ride it out. It took me 20 months from seeing my GP to getting surgery. Well worth the wait though!
As for which is best, neither. It depends on you, your eating habits, your willpower etc. Both are available on NHS, so it's just a case of reading and researching the different options and see how you feel. Near the end of the process, when you see the surgeon, he will advise on which he feels is best for you, but it's your decision. Read a few of the diaries on here, or blogs, for each type of surgery and see what they are really like.
Hope that helps, if I can be of any assistance just shout.
Ooh just remembered, there is a WLS support group that meets in Liverpool regularly, which is open to pre-op patients too, so if you want to meet people who have had the different surgeries and hear things first hand then let me know and I'll send you the details.
Shel