'Friendly' isn't as straight-forward as it seems. And it's only part of the equation.
I'm from the Midwest (where we're pathologically nice), and have been living in the Western Isles for 5 years now. People are very friendly, on a casual level. There are dozens of people that will say hello to me, have a chat with me in the shop, etc, and I've got friendly relationships with several colleagues. But there are only maybe a handful of people here (all over aged 65, and long-time friends of my husband's family) who I would say have actually shown any real interest in getting to know me. And it's the same for my MIL, who's only from one of the other islands, and has lived here 43 years.
So, on the one hand, the casual friendliness and courtesy is much the same as I'm used to. But on the other, it's still exceptionally lonely. And, of course, there's the downside to that casual, small-town friendliness: gossip, judgement and zero privacy.
The other thing to consider is what interests/activities/lifestyle your wife enjoys. It's always harder to make friends as an adult, but one of the best ways to do it is by getting involved in some activity; a yoga class, a church choir, community theatre, a running club, etc. For me, none of those things has been an option, because activities that interest me are nearly non-existent.
I've struggled to make this work, but recently had to admit that it's just not. The islands are simply not a healthy place for me, or my husband (and he's a native!). Fortunately, we're going to be moving to Inverness soon, and I'm really hopeful that it'll be a better fit for us. Still small enough to be relatively friendly, and low-stress, but big enough to offer a variety of activities and opportunities, which we can enjoy in anonymity.
So while I would say that, in the broadest of terms, the friendliest places in Britain are north of the Central Belt (or, at least, north of Hadrian's Wall), your wife's mileage may vary considerably, depending on who she is, what she enjoys, and what's important to her. And, the problem is, she might not be entirely aware of those things until after she's here.
Bottom line: take stock of what you/she really enjoy and value about your current life. Make your best guess. Then, be prepared to find out that you were wrong, and to move on and try somewhere new.