You are correct that you should not duplicate pages and swap out the keywords. Google calls doing so doorway pages. Doing that is against Google's webmaster guidelines. It could get your entire site penalized. As you point out it isn't good user experience and it could drive away customers.
You may create pages to target keywords if they have original content and would make sense for visitors. It isn't hard to think of ways to create pages for both areas:
- Separate pages for driving directions to the dentist from Dorset and Somerset
- Separate pages for testimonials of customers from Dorset and Somerset
- Separate pages for dentistry statistics and facts in Dorset and Somerset
It is also fine to write a single page that tries to target both areas. When writing titles for SEO you should:
- Accurately describe what each page is about
- Target keywords and phrases for which you want to rank by using them near the start of titles
- Avoid keyword repetition and lists of keywords
Your title B is awful. It is just a list of keywords with repetition. It looks spammy and it isn't going to help your SEO.
Your title A is better, but I doubt that it accurately describes the page. "Private Dentists in Somerset & Dorset" could be a good title for a page that lists all the dentists in the area. But in this case it sounds like this is the website of a particular dentist. For the website of a dentist in private practice, the title of every page should include the brand. The name of the dentist (or the name of the practice) should go at the end of the title.
It is hard to say exactly what the page title should be without knowing all the pages on the site. The home page should be targeted primarily at the brand name. It should be a separate page that targets other more generic keywords. I might suggest pages on the site like:
/
(home page): Dr. Phil N. Kavatee DDS, Dentist, 1 Main St, Somerset
/service-area
: Private Dentist Serving Somerset and Dorset - Dr. Phil N. Kavatee