Suggestions and Comments
I would look at it more as "Their English has been put on hold". It is much easier to re-learn than to learn and with your children's young age, they should have no problems after the first month or so.
I've worked with refugee children who came not speaking a word of English, and when they go into their grade level, work with an English as Second Language Teacher for certain hours of he day, they learn quite quickly. I taught them the little nursery rhymes and songs that children sing in class. I taught them the little games children play (“Duck duck goose). I read to them in English (You can purchase books on CDs in English...shouldn't be a problem in Japan.) Your children should listen to children's music in English (that you might consider mailing to them). They should watch childrens' DVDs/VCR movies in English (in fact, I would insist on English only for videos, not giving them the option of television in Japanese or videos in Japanese). It's important they play with English speakers, if that's an option. I would think there would be an American/English playgroup if your wife is in a fairly large city. She will not be able to speak much to the other moms, but your children will interact with other children speaking the language and see it's something their peers do, not just their Daddy on the phone.
Once they're in England, have plenty of playdates with children who speak English. In the beginning you might invite 2 children so yours will overhead conversation and the English kids won't be frustrated with not being able to understand. Usually there isn't the frustration that adults have, however, because children can play without traditional language. It's amazing really.
You could take NONE of the above advice and your kids will adapt. They're at a super age for a move, they have English in the back of their minds, they have incredible motivation to learn English (imagine not understanding what's going on in the playground...that pushes kids to learn quickly).
So, don't worry, children so young have suchflexible minds. You'll be amazed after just 1 month in England.
Proceed with confidence.