Which stores is the easiest place to purchase manga from?
Are there any supermarkets or main stores that sell them or can they only be bought from independent stores?
I would suggest checking if you have a local forbidden planet, they have locations all over the UK and always had a wide selection available; their website is here http://www.forbiddenplanet.co.uk/
I found most book stores generally have a small manga/anime section. Depending on what exactly you are looking for you may do better looking online, the best site in my experience is http://www.bookdepository.co.uk/ which returns over 100,000 hits when I search just on the word manga.
There are many other online options which you can find using a search engine.
I hope this helps.