And -- from a Briton -- just to compound the confusion, there is in the just-mentioned Gloucestershire, the small town of Cirencester. Several alternative pronunications known for this one: "sister", "sisiter", and "syre-en-ses-ter" (accent on first syllable in each case). The last of the three is, I believe, the most generally in favour nowadays.
That... would confound me no end. if you follow the same rules as Gloucestershire (Glou-ster-shire) then Cirencester should be Syren-ster not sis-ter! 
I think if I ever go to England, I'm going to have to make a cheat sheet of town names!
Rule 1 of English place-name pronunciation is, for sure -- there are no rules !
Re cheat sheets: the humorist Paul Jennings tried to create one such -- in rhyme, to boot. Part of it, follows.
...Tourists, attention ! Nor forthwith assume
You say as spelt each little town, like Frome.
The Welsh have funny names your tongue to twist with
Like Penmaenmawr and Gwlch and Aberystwyth:
That you expect. But when in England, how
Dare any tourist ask the way to Slough,
Who yesterday put someone in a huff
By thinking "brow" homophonous with Brough?
Though Glyndebourne opera for the cultured few is,
How can they get there but by way of Lewes?
Under the Sussex Downs, where snug they both lie
Are many other gems -- for instance, Hoathly.
What lord of maps, what orthographic jouster
Grapples with Bicester and its sicester Towcester*,
And never gets in something like a panic
If blank stares greet him when he asks for Alnwick,
Nor ends by thinking people downright spooky
Who make a place spelt Stiffkey rhyme with Newquay?
Tourists, you needn't be alarmed unduly
By England's treatment of a word like Beaulieu.
These Norman names (or Saxon ones, like Wrotham)
Unlettered men have modified to suit 'em,
And pundits long since gave up trying to teach 'em
How you should really say a name like Beauchamp.
Our names and views hold unexpected joy:-
Views English-Channel seaward, as at Fowey;
Views over sands, or rocky views and pebbly;
Pastoral views, near inland towns like Weobley;
East Anglian views, with churches old and mossy,
Each as unique as disyllabic Costessy*...
*A rhyming guide has the flaw that it can't cater for alternative pronunciations. Some people pronounce the T-place not like "Touster" as per the verse, but "Toaster", or "Tosseter"; and the C-place is alternatively pronounced "Cozzy".
