I know what you mean about the difficulty of finding real friends when, although you speak the language, you don't speak it like a native - you can't really be yourself when you are far less articulate than you are in your native tongue, you feel slightly slow and stupid...
However what makes you so sure you'll find real friends in your northern village? It tends to be a lot harder to make friends as a part of a couple and family unit in your 40s than as a social butterfly in your early 20s or younger! Most people in small northern villages probably already have a friendship circle, possibly dating back to their own school days and involving blood relations like cousins as well as people they've known since they were at primary school themselves, people they met when they all had babies at the same time etc.
You may unfortunately find that although you have the almost physical relief of living in your native language, you remain in a situation where, for different reasons, people are friendly but not friends...
You will always be an outsider now, having lived abroad for a long time, you will always be "other"... if you are very unlucky or don't try hard (use the local school, join local clubs, go to the local pub, don't get bitter and imply to your kids that your family are different due to your wider horizons when your kids aren't getting invited to parties because all the other mums are long term friends and the kids have all known each other since they were babies and most of them are some convoluted type of distant blood relation to one another) so will your kids...
I know absolutely from personal experience where you are coming from, but think you may not be being realistic about what returning to the UK - especially to what might be rather a closed community where everyone will probably be friendly but not looking for a new friend from "outside" - would really be like.
I also think it may well be considerably worse for your kids to return to the UK than to stay where you are - my kids are firmly part of our local community and bilingual too, and I know their worlds would contract and suspect they'd be less well accepted in the UK (I remember even being an "incomer" from "down south" in a northern English village as a child and we were never really totally accepted as locals, we were always "incomers" / "not from round here", other, outsiders...)