Gloucester is Gloster. Worcester is Wooster (and so is Worcester Sauce). Bicester is Bister and Towcester is Toaster. And that’s just to start. If you’re visiting Beaminster or Leominster, remember to call them Bemster and Lemster. But if you pronounce Cirencester the way it is spelled you will be correct.
When you take a boat ride down the Thames (Tems) to Greenwich, remember to call it Grennitch. And of course Norwich is Norritch. If you should drive thru Slough (it rhymes with cow) to Windsor Castle, call it Winzer Cahsle, please. (The upper class (clahss) English love to use the ah sound to whatever they can. It distinguishes them from the plebeians.)
Further out in the country, you may visit Tintwistle (Tinsel), Oswaldtwistle (Ozzul-twizzle) or perhaps the tiny village of Great Barugh (Great Barf). You can also go to Torpenhow (Trepenna) and climb the hill. Since tor means “hill” and pen means “hill” and how means “hill,” you’ll be standing on Hillhillhill Hill.
In Wales, everything is a verbal trap! Llandudno (hlan-did-no), Pwllheli (Poo-heli), and Ponciau (Ponky) are just three samples. And Scotland is worse. You can visit Kirkcudbright (Ker-coo-bree) and Wemuss (Weems).
On your way back to London (Lundun in the south of England but Lunnen in the north), you may pass thru Prinknash (Prinnish). If you visit Cambridge University, remember that Magdalene College is pronounced Maudlin and Caius College is pronounced Keys.
Some English family names can also be trap. The most famous best examples are Cholmondeley Chumly) and St John (Sin-jin). But there are also Belvair (Beaver), Death (De-ath), Warwick (Worrick), Ewell (Yule), Beaulieu (Byoo-lee), Fowey (Foy), Ruthven (Rivven), and Meikleour (often pronounced McClure).
Finally, if you’ve studied literature, you may know that the “fair youth” of Shakespeare’s sonnets is Henry Wriothesley, 3rd Earl of Southampton. Henry’s surname is easier to pronounce than spell: Risley.
Have a nice trip.