I think the main reason is that China has been exporting people and their cuisine for hundreds of years. Japan, for the most part, has not. I think it may be partially also because Japanese cuisine depends a lot on fresh seafood. Until recently, it wasn't so easy to get some of this stuff even if there was a Japanese community that wanted it.
Potatoe, there are many japanese restaurants that offer takeaway food in London. London has Europes largest population of japanese. Where in Britain do you reside? If you are anywhere near London, it shouldn't be a problem.
It is strange, London does have Europe's largest Japanese population, yet I rarely see any. Japanese restuarants on the whole are quite rare, never seen a pure takeaway one, i'm sure most restaurants allow you to pick your food up though.