What is the best way to travel from Bath to Aberdeen? Plane? Train?
Train. Without a doubt. Flying is too much trouble. I always drive, so can't help much. Somebody else will tell you exactly how to do it.
Fastest would be to take a train from Bath Spa to Bristol Temple Meads, a bus to the Bristol airport, and an Eastern Airways flight to Aberdeen.
Walk-up fares for long distance trains in the UK are expensive. If you book well in advance at www.eastcoast.co.uk, you can get a discount fare for Bath Spa-London Paddington combined with London King's Cross-Aberdeen. You would be on your own to get from Paddington to King's Cross by either Tube or taxi.