I understand, OP. I have lived in my area for 15 years and haven't made one friend - and it's not for the want of trying! People always seem to have old friends and have an almost visible "no vacancies" sign hanging round their neck.
I do have dcs, and have made mum acquaintances, but 99% of the time all you have in common is children of the same age, and if people move you don't stay in touch. And as you observe, family is priority, especially when people have young children.
I agree that a lot of activities seem to attract (quite a bit) older people. I investigated the WI and they said I was welcome to come along, but the youngest current member of the group was 80! Other bad activities are ones where you pluck up courage to go along and people are there with their existing friends . I am still sweating with embarrassment at the memory of the book group I went along to at the library where people kept asking me to move seat until there were no more seats and when I said that, there was glaring and huffing - and then the woman doing the coffee packed up as I approached. [Not helpful, OP, I know - but I'm still shuddering at the experience!]
I know that East Anglia can be a little challenging on the social front, as most of it is rural. But I would second giving MeetUp another go, and definitely try a sporting activity, even if you are not sporty.
Oh, something that I know does bear "friend fruit" is following a band. Especially if it is a crappy band, or a tribute band or whatever. Then it becomes a social activity rather than about the band. Someone I know goes all round Europe following random band with a group of die-hard fans.