Yep, he's way way eligible for filing in the I-130 with London USCIS.
Something to think about in advance is USA domicile as he's been in the UK for so long he may be asked to show more than the usual amount of evidence. Does he have any property in the US still? Don't worry if he doesn't, it's just that would be the easiest way to prove USA domicile.
Has he been filing his US tax returns (assuming he's been earning money in the UK)? If not, don't panic. The IRS will help him backfile and unless he made over $82,000 USD in any give year, he won't owe any money on his UK income.
Also, if he doesn't already, he should open a couple US accounts in advance if possible. And lastly, once you're getting closer to the interview, securing a lease or mortgage in the US would really prove USA domicile.
I hope that's helpful instead of making it sound too complicated! It's really not too bad - just something to be aware of and prepared for.