OK, I didn’t make myself clear. What I meant is that the modern pronunciation isn’t directly aping French. It doesn’t replicate French pronuciation in the way a modern English word such as ‘cafe’ does, but has distinct English hard sounds quite unlike the French ‘St Jean’, and it simply happens to have distant French origins.
Sorry for the double-post, but I meant to include that this was why I mentioned Four Weddings - it’s an example of somebody unfamiliar with the name, who attempts a ‘French pronunciation’, and sounds (a) wrong and (b) silly.
The Federation diplomat played by David Warner in the lamentable Star Trek V: The Final Frontier is named St. John, and it’s pronounced “sinjin.”
Another odd British pronunciation: Cholmondeley is pronounced “chumley.”
It wasn’t until I saw the Tales of the City mini-series that I realized the name “Beauchamp” was pronounced “Beechum.” 
One doesn’t read about Airwolf, one watches (or watched) Airwolf! 
And, IIRC, that character was Stringfellow Hawke’s missing brother – though I never knew that “Sinjin” was spelled “St. John” until this thread!
For what it’s worth, I’ve always understood the capital of Newfoundland, St. John’s, to be pronounced “sinjahns” (although I haven’t lived there since I was 5, so YMMV).
Okay, well, if we’re going to go down this road.:
Blackley = Blakeley
Cholmondeley = Chumley
Cockburn = Coburn
Costessey = Cossey
Dalziel = Deal
Davies = Davis
Featherstonehaugh = Fanshaw
Happisburgh = Hezbra
Keighley = Keithley
Magdalene = Maudlin
Marjoriebanks = Marchbanks
Towcester = Toaster
Woolfardisworthy = Woolsey
More:
Caius = Keys
Ralph = Rafe
So is Worcestershire correctly pronounced “Wuss-ter”?
Wuss-ter-shur ifits the County
Wusster sometimes for the sauce- the shur is just dropped for no apparent reason.
Beaulieu > Bewley
Warwick > Warrick
I remember being terribly confused the first time I saw A View To A Kill. Even so, I had a high-school teacher with this surname who pronounced it ‘Saint John’.
(Around here, Sault Ste. Marie is “soo saint maree”, or just “the soo”.)
“Sin Jin”
Wonder what jinwicked would think about that.
Belvoir > Beaver
Fotheringhay > Fungay
I know how “Sinjin” is spelled because of that show! In one episode, some people claim to have found String’s brother’s body. (It turns out to be false.) The coffin said “Hawke, St. John.”
I almost forgot. Isn’t there a British detective series called Dalziel and Pascal (or something like that) where the first title character’s name is pronounced “Diel”?
I remember one episode of that show where they were interviewing a guy who had the surname St. John. The guy insisted on pronouncing the name “Sinjin.” Eventually the cops find out that the guy is a Satanist, whereafter they very carefully address him as Mister Saint John just to be jerks 
Yes, Dalziel and Pascoe, pronounced Dea-el and Pascoe.
Another one: Hawick - Hoik
Is this like pronouncing “Belvoir” as “Beaver” here in Belfast?
Aah, yes, the chain-smoking, sweet-tea-drinking chimp from Durrel’s Overloaded Ark was my first exposure to this oddity.
Or in the UK, Harry Enfield’s Mr Cholmondeley Warner, warning women to know their place.