The uneducated East End of London accents of the 1950's and Early 1960's might explain some of it but not all of it, because I used to hear a distinct difference between many Jewish speakers many of which my parents and I lived with back in the day. "This is the only flat available if you don't take it, you will remain homeless". Since that time I now know that there was more than one type of Jewish speaker I was hearing during my formative years. From more than one region of Europe and from either one of the two major Jewish whatever religions sects etc.
I wasn't blaming Americans just noting that bagel or bagels has become the standard way of writing it and saying it now, except on any original shops older than forty five years.