I'd generally avoid messing around with trains and bike boxes if you can. It'd be a bit of a nightmare schepping from the airport into a city centre then catching a train - so if you were just coming for the cycle, I'd fly into Inverness, although it's not the easiest connection.

However, if you wanted to combine the trip with a city visit, you could go to London for a couple of days either end, and get the Caledonian Sleeper. Or into Edinburgh which is also well worth a visit.

If you wanted to combine with a bit of a drive you could fly into Glasgow and rent a car and drive from there over a couple of days. It's pretty scenic up the West coast. Or you could do a loop... Glasgow - Inverness - Edinburgh - (Glasgow). You could probably do one way car rentals as well.