pronunciation - How is "erm" pronounced in the UK, and why is it spelled that way?

I see the interjection "erm" written in internet forum posts fairly often, and I have occasionally seen it in British novels, in opinion pieces and articles on cultural topics in newspapers and magazines penned by British authors, and in British film subtitles.

How is "erm" pronounced in the UK, particularly in the south of England?

Unlike, say, "uh" or "um," which are listed in most of the online English dictionaries, "erm" as an interjection is largely absent from the dictionaries.

I think Google.com's (American-influenced) translator pronounces it somewhat like /ɜːm/, similar to the American pronunciation in the recording linked to in the Wiktionary entry.
Google.co.uk's translator gives a more non-rhotic pronunciation.

These Youtube recordings linked to below, which purport to demonstrate how the word is pronounced, sound broadly similar to the Google.com "American" pronunciation, with a rather pronounced r influence on the vowel:

But the pronunciations I have heard in films where the subtitled spelling was given as "erm" have always sounded much more like the American pronunciation of "um." An example of that is in most of the pronunciations in these outtakes from interviews of the actor Emma Watson.

Almost no native American English speakers use "erm" in practice in the US, so it does not seem very useful for the guides to provide the supposed American pronunciation for this word. (A search of the Corpus of Contemporary American English finds about 160 uses of "erm" (case-insensitive), almost all of those being for the acronym "ERM." By contrast, a search of the British National Corpus finds over 60,000, with most of those being of the interjection (from a quick glance).

So the questions are:
- How is "erm" pronounced in Britain, especially in the south of England?
- Roughly when was this spelling of the word widely adopted in the UK?
- Is the current spelling based on an earlier rhotic pronunciation that has largely been abandoned?
- Or was it, perhaps, a contraction of "er, um"?

I see that someone claimed in the discussion on Wiktionary that the "explanation of how "erm" and "um" are distinct is completely made up. They are the same thing and are pronounced the same. "Erm" is simply the British spelling and "um" is the American."
If that is the case, why is the British spelling not "um"?