Oh it was regular, right enough. Just not very frequent!
Regular - arranged in or constituting a constant or definite pattern, especially with the same space between individual instances.
Whilst it had a definite pattern, certainly not with the same space between individual instances. As such it's reasonable to argue that whilst during the summer months there was a definite pattern over the year it wasn't regular.
Would anyone say Newquay gets a regular service, yes, would anyone say that Newquay has a regular service to anywhere other than Par, almost certainly not. As such if GWR announced that Newquay was, for the first time in ages going to have a regular service to London (say twice a day all year round) few would quibble.
Likewise if SWR had said there was going to be a regular (say twice daily) service to Corfe Castle in 2020 (after running the service on Saturday's during 2019), then again few would have quibbled.
Technically a weekly bus on market day, one service there and one back is regular, however again if that was made so that it was useable by commuters the bus company could justifiably call that the first time it was regulator.
Therefore it could be argued that the service to Okehampton was regular, however most would accept that an at least a few times a day most days of the year would be the minimum that they would expect if a public transport service was called regular.