No, “ch” is so much more distinctive a sound than a sibilant “c” that there’s that much less risk of elision. </pedant>
Now, if you were talking about Slaithwaite or Shrewsbury…
No, it’s pronounced “Casterbridge”
(only kidding - that’s Thomas Hardy’s fictionalised name for it - although it has become sort of currency for the naming of guest houses, etc in that town)
Could be a regional thing perhaps. I’m very accustomed to hearing people call it as I described above.
Ok, so Worcester is pronounced “Wooster”, Leicester is “Lester”, and Gloucester is “Gloster”.
Why, then, are cities like Manchester pronounced phonetically? Shouldn’t it be called “Mooster”?
As** Patrick London** pointed out, there’s one very obvious difference between the first three, and the last one…
No, it’s just an old joke. I lived in MA for a couple of years and can even pronounce Peabody correctly. 
Well, all you non-Wooedhouse fans, how do you pronounce Psmith?
Exactly plus one. In my 30th year of re-re-reading Wodehouse, Hugh Laurie’s portrayal is on the money.
It just sounds like a colloquial shortening to me. I’m sure we must have some example of this in American English, too, but I’m blanking.
On the bag of Walker’s crisps, it’s Worcester Sauce, indicating to me that it’s a nickname or alternate name for the full Worcestershire Sauce. That is, it’s not Worcestershire that is pronounced as Wooster in the sauce name, but rather that it is shortened to Worcester, thus the shortened pronunciation.
I think that might be because the crisps do not contain any actual Lea & Perrins product, so they’re using an alternate name for an ersatz flavouring.
Could be. In the US, any brand can use the name.
Also, Oxford Dictionaries has an entry for “Worcester sauce.”
Further research suggests that Lea & Perrins did try to sue Holbrook’s back in the early 1900s over the use of the term of “Worcestershire Sauce,” but lost. However, they did get rights to the phrase “original and genuine.” So it sounds like anyone can use the name, but choose the more colloquial “Worcester,” since that’s what most people call it anyway.
Uh, that may be true in the Midwest, or the West, but down here in the South, it’s much more “English” like.
I grew up about 60 miles from LAN-CAS-ter, California. When I lived in Ohio, I was not far from LANG-kes-ter, Ohio. Now, I live just 20 miles north of LANG-ster, South Carolina. Where, if you ask them to do so, they will put ōl in the car, so you can go to the low-country bōl.
Conversely, of course, my pastor’s name is Chip, pronounced around here as three syllables, especially if it’s your mom and she’s not happy with you and wants you home! 
Because “chester” and “cester” are not the same thing.
Thus, Winchester is pronounced similarly to Manchester (not the same, because Manc is not identical to the dialect of Hampshire). If they were pronounced the same as Leicester is, they would be Manchster and Winchster, which doesn’t exactly slide off your tongue the same way.
It’s a matter of shooling.
It’s called slovening (a term I love by the way). A lot of people saying something a bit difficult for a few hundred years and it becomes smooshed.
There are other technical terms but my daughter the linguist isn’t here so I cannot consult her.
What I think is hysterical is what happens to French place names in the US. I remember trying to get someone to tell me how to find Calais, Maine. “Oh, you must mean Callus!”
And then there’s the town I grew up in, Sanazay, founded by Spanish Franciscan missionaries and named for the sainted husband of the Virgin Mary.
There’s Hyooston texas and howston st, which is near grennich village not to be confused with green witch Ct.
There used to be even more odd shortened pronunciations of English towns which have (sadly, IMO) been dropped: Cirencester apparently used to be pronounced as “Sissiter”, and Pontefract was “Pumfrit”.
Locals in a village I used to live in added an entirely superfluous extra schwa in the middle of the name. I think it basically served to distinguish the folks who weren’t from round 'ere.