U.K. regional anomaly in pronouncing -ought

I'm reading Henry Fielding's Tom Jones, written in 1749 and based in Somersetshire. I'm intrigued by Fielding's unusual but consistent representation of the way some less-educated characters pronounce "thought" and "ought", as evidenced in this passage (the italics are in my printed edition, presumably intended by the author):

"Ay! Ay! Good lackaday!" said the landlady. "Who could have thoft it? Ay, ay, ay, I am satisfied your honour will see justice done; and to be sure if oft to be to every one. Gentlemen oft not to kill poor folks without answering for it. A poor man hath a soul to be saved as well as his betters."

Fielding puts this pronunciation on the tongue of a number of different characters so I can only assume he is trying to capture accurately the local dialect, but to me it seems an unlikely aberration, since it involves not merely the change of velar fricative [x] to /f/ but also the change of vowel sound from /ɔː/ (as in jaw) to /ɒf/ (as in off).

My question:

Was this pronunciation peculiar to 18th-century Somersetshire, or was it also common in other parts of England at that time? Does it still continue as a regional (or perhaps social class) dialect in the U.K.?

NB: (1) this is not a question about the novel (which would belong on our other site [Literature.SE]) but about the regional pronunciation it purports to reveal. (2) herisson's comprehensive post on the diverse pronunciations of "ough" is relevant (and an interesting read!) but doesn't specifically address the regional/dialect anomaly.

Solution 1:

The "f" sound is still used in "laughter", and was acknowledged in an 1875 book "Report and Transactions - The Devonshire Association for the Advancement of Science, Literature and Art, Volume 7" as a Somerset, Devon and Cornwall pronunciation. This was in the section on "Verbal Provincialisms in South-West Devonshire"

The entry starts by saying that "Oft" is used for "Ought" in Torquay and Ashburton, and goes on (if you follow the link) to mention

• thoft = thought
• thof = though
• oft = ought
• auft = aught
• sife/sof = sigh
• sift = sighed
• dafter = daughter (very true in the case of my own batty offspring)
• boft = bought
• broft = brought
• loffed = laughed

I'm not aware of anyone still using this pronunciation, other than in "laughter" of course, but I wouldn't be surprised to find some more rural, elderly folk from Devon, Somerset or Cornwall still using it.