The electoral roll is for private individuals to view and I believe that it will provide DOB information. Try the local council offices as your first port of call.
Electoral roll does not show a persons date of birth. This information is only asked for if occupants of a household are approaching their 17/18th birthday and if the voter is over I think 85 because they are no longer eligible for jury service.
I have to ask, why would you want their date of birth? Surely, you should ask them?
If they have ever been a director of a company.. maybe even set up their own company.. you will be able to get their date of birth from information held in Companies House.