No progress yet in verifying the MoD's account of 18" naval gun firing onto Dartmoor but a couple of observations can be made that make this maybe a little more likely to be a myth.
Firstly, there was certainly a station at North Tawton, about 1 mile south of the village. The old station building and the Railway Inn remain. There were sidings there, though they (and the main line) run east-west, whereas the firing direction would have been a little west of south. The guns weighed 190 tons, so is it really likely one could have been mounted on a railway wagon that was able to traverse through 90 degrees and that wagon and mount would withstand the shock of firing?
Secondly, although the Okehampton range as we know it today would be well within the gun's range (its 8.5 miles for North Tawton station to Okement Hill, and these guns were used in action in WWI at over 20 miles), the first 4.5 miles is over open farmland plus the villages of Sticklepath, South Tawton, South Zeal and Belstone. The results of a mis-shoot would potentially have been catastrophic.
As for Halwill Junction, this is further out, at 12.5 miles from Okement Hill. Two-thirds of the line lies over farms and villages, not the moor or the range. The railway at this point is aligned on heading 162 Degrees, so not as far adrift as at North Tawton, but a shot along this direct line would miss the moor altogether. Plus, this is the main line, there are no sidings here I can spot on the old OS maps.
One thing seems clear. The two 18" guns that were installed on Furious in WWI, and later used in action in 1918 when installed on two monitors, cannot be our gun, as these were both scrapped in 1933. Gun 3 was used at Silloth for cordite proofing tests in 1920 and afterwards converted to a 16" (40.6 cm) design between 1921 and 1924 for use as a prototype for the 16"/45 (40.6 cm) Mark I guns destined for the Nelson class. It was then used for various trials until 1942 when it was sent to Woolwich where it remained until scrapped in 1947. So Gun 3 is the only candidate and was certainly used for firing trials of various sorts in this vintage. But would the War Dept really have moved all of its 190 tons to Dartmoor in the hard-pressed days of 1940-41 for tests that could presumably have been done where it was?
So far, I say this is a myth despite what the Mod say.