It's almost impossible these days. You could, of course, try the BT Phone Book:
https://www.thephonebook.bt.com/person/
However well over a half of all residential numbers are ex-directory these days. (It's not going to be easy either if you're looking for Karen Smith but the phone line is registered in her husband's name, simply as 'J Smith').
Googling can, very occasionally help if, for example, the person you're looking for has their contact details listed online because they're the secretary of a local club or society.
If the person that you're trying to find is someone who'd like to hear from you (rather than, say, a former tenant who owes you money and who's trying to avoid you), looking for them on Facebook, and then making initial contact there, is certainly one possibility.
Beyond that, the only online way of finding someone is to look for them on 192.com which, if they're listed there, involves paying a fee to get the information that you're seeking. Even then, you need to remember that 192.com only uses the 'edited' electoral register, showing people who've not ticked the box on their registration form to have their contact details removed. As many (most?) people tick the box, in order to cut down on junk mail through their letterbox, there are millions of people who simply aren't listed on 192.com.
If you happen to live near to the relevant town, you could go to the council offices to view the complete electoral register. However that's not easy as
(a) you need to book an appointment ;
(b) you're not allowed to make any notes or otherwise write down an address ; and
(c) the listings are only in street order and not in alphabetical ordrr of people's names. (So you'd need to look at everyone living on Aardvark Road, followed by everyone living on Abbey Crescent, etc, etc)