These people are highly paid and if i was Boris I would be expecting some return on the taxpayers investment. Having them sit (or more likely stand if I understand parlous state of UK rail today) on a train both ways , while satisfying the wishes of the proletariat, would hardly be conducive to them concluding important Brexit discussions and then getting on with other (no doubt Brexit related) tasks on their return. That's what you pay them for. Attending a whiskey reception is clearly political obfuscation designed to cast a dim view of actions taken without revealing the real reasons for those actions. The article is dated 31 Jan (I'm assuming that's 2020), so this is related to something quite urgent in relation Brexit and the PM's requirement to 'Get Brexit done'.
The RAF (if it is like our service), have a certain amount of allocated flying hours for exactly these purposes and the choice is always that they be used in this manner or the squadron simply bores holes in the sky. We all know there is far more training value in these already budgeted funds being used on 'real' tasking. I'm calling fake news on this one and if the opposition was in power, apart from the fact they would be Remaining rather than Brexiting, they would do exactly the same thing. In my country I have seem far clearer examples of politicians using our Air Force for a taxi service from their home electorate to attend parliament because..well because they can.