Non-independent stores in the UK that sell manga 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.