I think what makes a good reference for a British academic post isn't very different from the American standard, at least judging by the many American letters of reference I've seen for candidates. One persistent problem, of course, is inflation. Reference-writing culture generally has become so soaked in hype that you are forced to layer on ever more superlatives to avoid it looking as if you are damning a candidate with faint praise. As a result, you are always courting the opposite danger, of praising someone in such ridiculously overblown terms that what you say will be dismissed as incredible. My sense is that British norms are a little less hyped up than American ones are. If you can just occasionally mute your praise of the candidate, to indicate that you are being measured and thoughtful, then it is more likely to render the rest of what you say credible and less likely to look like a career-ending doubt than might be the case in an American setting.
That said, of course, a single sentence of actually negative comment, or indeed of barbed or studiedly ambiguous phrase, can of course be enough to sink a candidate. So can writing a reference which is very short or very bland. (Around two pages is the norm.)
Naturally, the best thing to do, where possible, is to cite actual evidence proving your case: comments from examiners, student evaluations, or whatever. Much of this may appear in the candidate's CV, but tell us anyway, partly because we can always miss stuff buried in a long CV.
On matters of substance, different British institutions look for different things. For some posts and some institutions, research will be more important than teaching; in others, vice versa. Naturally someone who is excellent in both fields is what we all want. But things which might particularly tickle a British appointments panel would include:
1. On research: is this person productive? Our system, sadly, has little place for the brilliant scholar who produces one superb book every 15 years. We need a regular stream of high-quality articles and monographs, meaning, normally, a book every six years or so. We want to see evidence that someone can churn the stuff out.
2. Can they attract external funding for their research? Our system increasingly emphasises grant-hunting. Appointments panels like scholars who have a record of doing this, and / or who can be shown to be energetic and creative in attempting to do this.
3. Can their research have 'impact'? Without getting too deep into the horrific entrails of the UK government's system of research funding: if the candidate's research has the potential, one day, to make some kind of tangible, beneficial change to the world outside the academy, we like that. Writing books that lots of people like to read is not, in itself, tangible, beneficial change, though it can be a start. Actually changing the ways ordinary people, or churches, or charities, or governments, think or behave - and doing it through the originality of your research: that's what counts. If people have actually done this, great. But what we're really interested in is their potential to do this. So if there is a story that can be woven here, do so.
4. Will they be a magnet for doctoral students? The PhD economy works very differently in the UK from the USA: many doctoral students are self-funded, and universities generally want to try to attract as many as they can. So someone who has the potential to bring a large flock of them in, especially ones from outside Europe who pay a higher rate, will set the cash-registers ringing.
5. On teaching: especially for junior scholars, they may have lots of teaching on their CV, but how much independent experience do they have? This is often difficult to discern, especially for panellists who've not worked in the USA and don't know the system. What we like is someone who has experience of designing and delivering entire courses on their own initiative; and someone who has experience of supervising student research projects. It would be useful to emphasise anything of that kind that you can.
6. Do they like students? Not all academics do. But even those who don't will often respond warmly to the young and naïve who still do. There are academics whose research is their life and whose teaching is their chore. Better to give the impression that this person will not be like that.
7. Collegiality. Is this someone who will muck in? If they are handed a tedious but important administrative job, will they do it cheerfully and effectively? Will they be patient committee-fodder? Tell us with a straight face that the answer to all of these questions is yes.