Let's say you have a PhD, and they have a PhD, and you are familiar with each others' work, but you never met personally. Would you address them as "Dear Dr. Smith" in an email, or "Dear John"?
A further source of awkwardness: Suppose you are addressing several people at the same time, and some do not have a PhD yet, do you write "Dear Dr. Williams, Mr. Johnson and Dr. Smith"?
Dear Barbara (if I may),
Xyz.
Kind Regards, Carla
Don't ever Mr., Ms. or Mrs. (urgh) anyone because it gets gross quickly for anyone who is not cis, and you don't want to assume their gender.
The hurdles to address someone by first name are very small in British academia. Being mutually acquainted with each others papers, or eg being introduced via a common acquaintance, are definitely enough to make using the first name acceptable.
If you are using a formal mode of address, you should get it right though. That indeed means using "Prof" for professors (associate professors are not professors in the UK), using Dr for everyone else with a PhD, and Mr/Ms for those without.
Generally, "Hi John".
If you're in a formal setting, they're particularly accomplished, and it's the first time you've met them, maybe "Doctor X" but that would generally be the exception and, in my experience they will almost always tell you to use their first name.
If you're emailing, having never spoken to them before, Dr X might be more appropriate for the initial email and then go with whatever they respond with from there.
Now, you might get the odd stuffy one but in my experience next to nobody cares and most people are more than happy with just being called their name.
If you've never communicated with an academic before, best to play it safe and address them either as Dr. Lastname or Prof. Lastname. Be absolutely sure to check which title they use by going on to the university or institution website (professor is an earned title in the UK, and most academics are not professors). If they don't have a PhD (they might be a student) then usual title conventions apply.
Include your full name (including your title) in your message if communicating in writing. If they address you with your first name and/or don't use their title in their response, then you can safely assume, at least in writing and one-on-one conversations, that you're on first name terms now, and should from then on communicate as such. I find most academics tend to prefer first name terms nowadays.
One important thing to note is if you're conversing with them with an audience. Some academics will be happy for you to be causal with them privately or with a group of other academics, or perhaps even PhD students, but they will be bothered if you don't use their title when introducing them or conversing with them with a group of undergraduates listening in, for example.
