Don't think this is quite what the OP means by 'decimal miles', but remember that the West Berskshire tariff card had decimals of yards, but they were trying to get rid of them:
West Berkshire Council wrote:
Tariff 1
Applies between 06:00 and 22:00 Monday to Saturday.
Initial distance not exceeding 377.1429 yards or part thereof and/or initial waiting time
of 81 seconds or a combination of time and distance £2.80
For each subsequent 125.7143 yards completed or part thereof 15p
Waiting time: for every period of 27 seconds or part thereof 15p
viewtopic.php?f=2&t=35116&p=412217But whether that was actually programmed into the meters or not isn't clear - I'd guess the meters couldn't accomodate that degree of accuracy, and the numbers were rounded. If I recall it correctly, when the Digitax agent manually programmed my 20-year-old meter, it would only accept whole yards, and that the kind of numbers on the West Berks card couldn't be accomodated.
But whether they'd accept 'decimal miles' (I'm guessing that means 20p every 0.13 miles, for example), I'm not sure, but I suppose it would depend on each meter manufacturer.
But like everything else, I'd guess the most modern meters could be programmed to accept decimals of miles, and it would just be a case of the meter manufacturers asking their programmers to amend the software accordingly.
And, of course, assuming meters could accomodate the likes of the West Berks figures, it shouldn't be that difficult to convert a decimal mile into a decimal yardage.