There is a massive difference between a sleeve and a band. Bands do not work. They have a 84% failure rate over 10 years and are rarely seen as being clinically wise now because of that. I think in most areas the NHS wont even do them. I'd be wary of a private doc doing them for this reason. Sleeves and bypasses are statistically proven to work. All the pros and cons below apply to a sleeve and not a band.
I had went the private route.
I know a couple of people who got it on the NHS. One took a year and a half, the other two years and a bit. They had to jump through quite a few hoops they thought were patronising and no real help but decided to suck it up to get the surgery paid for.
Mine cost 7k. In Europe, not UK. Went abroad as I wanted a very specific surgeon and that is where he was at the time.
Gang of 8 of us out there stay in touch. We are all in a healthy BMI range now. Took between 12 -18 months for us depending on starting weight. You lose about a stone a month for the first few months.
I have about 7-10 vanity pounds I struggle with.....my weight stabilised at a size 12 but to look great in clothes I need to be about 10 pounds lighter than my body likes, so I have a bit of a yo yo from a 10 to 12 which does my head in, funnily enough.... bearing in mind I was 20 stone and couldn't walk up a flight of stairs its a high class problem to have 
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Negatives: its surgery. its putting yourself in harms way. Anaesthesia risk (smallish but there) Infection (bigger risk and potentially fatal). Saggy skin that you may not feel able to live with without more surgery (so more risk and ££).
You will never be able to eat whatever you want ever again. Either as your stomach is too small for big portions (or even adult meals) or due to dumping syndrome(where your body can't take sugar any more and you get the shakes, diarrhoea etc (I don't have this, but a lot of people do). You will need to take vitamins and omeprazole for life. Issues with nutrition due to malabsorption of vitamins, people often need b12 shots every 3 months, for example
Mentally its a complete rollercoaster. I don't think anyone ends up morbidly obese without a massively complex relationship with food.....that still has to be unpicked. And its really not easy to take away your only coping mechanism and then do something thats a massive emotional upheaval like halving your body weight without having the coping mechanism. Suicide rate and divorce rates are high in post surgery patients.
In short, if there is any other less drastic method you think might work for you, try it first. If you have no options left, get the surgery because obesity will kill you otherwise. And you'll probably spend years hating yourself first.
Trying to give you a really honest view of the pros and cons here. I'd choose to do it again because it saved my life, no doubt.