I agree that us Norwegian are a bit reserved, but like you, OP, so are many brits.
I've heard immigrants here saying it is hard to make friends, but when you do, the friendship goes deep.
I've lived in the UK for a few years and struggled a bit to make friends. I didn't make any proper friends in my uni classes or in halls. But I joined a mountaineering club, and they were very welcoming, and I am still in touch with some of the people there.
So my recommendation to people moving to the UK or Norway is to find activities where you meet people you have something in common with.
For the kids it shouldn't be difficult, though, but good idea to have them join scouts or football or something like that, especially if they go to private school, so that they get to know local kids.
Kids learn English early here, so even 8 year olds will probably understand some English, and 11 year olds more so, so it should be possible for the kids to communicate on a basic level with other kids before they start learning Norwegian.
Almost all adults speak English and are happy to do so one-to-one, although some might be shy due to small vocabulary or heavy accent.