A shorter pool gives you more practice at turning however you will need to get used to the feel of the 25m pool. Sometimes leisure centre pools get very busy and it is nigh on impossible to train in them though! You can't win either way, what you can do however is work on your stroke technique and try to build up some stamina no matter what the length of the pool is.
How about Sportshoes.com, admittedly I haven't looked up your shoes but I have bought from there at a decent price in the past. There is an Asics outlet in Gloucester Quays, definitely worth checking your nearest outlet village which may be Bicester?
A bit of background, I used to weigh 16.5stone and be a size 24 and lost 6 stone to a size 10, after having a child currently 11,5 stone and size 12/14 trying to get back to where I was! It really was a 2 pronged approach where I changed my diet and did some exercise.
Covering exercise first I started Spinning first and then started running after I had built up some fitness and lost a bit of weight, I still Spin now. WIth running it really is a case of slowing it down to a pace where you can hold a conversation, it may be slow but that really does not matter! I have always preferred running outside as I like the scenery, quite honestly I can't run on a treadmill as I hate it and really struggle as a result. Nowadays I do triathlon so am swimming, cycling, running, Spinning and practicing Yoga.
On the food front it is best to plan ahead where possible, take packed lunches and snacks with you to work and keep a food diary. If you wish to try something more structured rather than doing a fad diet like 5-2 I would suggest good old fashioned Weight Watchers where you do not have to fast or give anything up you quite simply eat everything in (structured) moderation.
Most of all I want to say well done for starting, the first steps are always the hardest and wish you good luck with your endeavours. You will get there and it will be worth it!