I did LEJOG this year and agonised over getting back for some time and did plenty of research. Europcar do one way hires from Wick Airport for around £80. I've never had any problems getting my bike (61cm frame) and the wifes bike in any car and we regularly put both in our Vauxhall Meriva with seats down and wheels out. The other alternative I seriously looked at was a one way hire car from Inverness after catching the train from Wick. In the end I booked the car from Inverness with Enterprise because they did unlimited mileage (520 miles back to Newark on Trent) for £60 and £55 in fuel. I booked a small car but can't remeber which make it was. I arrived in Wick a few days earlier than expected and booked tickets on the train from Wick for the next day. There were four trains a day and they had spaces on 3 of them, albeit they only have 2 spaces per train. This was in June.
If it were me alone I would use the same method again; however, for two people I would book a one way estate car from Europcar at Wick Airport. Simple, leave when you want, almost certainly cheaper than train, no trains to change, no worries over whether the bike is actually booked or whether you can get a seat and no idiots to sit by on the train. Gets my vote every time.