What salutation to use for someone with a PhD who is a Lecturer or Reader in the UK?
The person's title in this case is (almost always) Dr, so "Dear Dr Last-name" would be the standard salutation for a first correspondence.  If they sign their reply with their first name, then you can consider yourself welcome to write "Dear First-name" in future.

Academics in the UK are very used to receiving email addressing them as Professor, so it's not a big problem if you do this, but there is probably a slight increase in the chance that they might view your email as spam (I say this because the overwhelming majority of emails I get which address me as Professor are at least somewhat spammy).
If someone's title is Professor, this will always be listed on the university website, so if you've done your research and found out that they are not a professor, it's probably best to use Dr.