I think it is cheaper and quicker to just fly to Southampton then get a train to Totton train station and bus it to Dibden.
Below is how I ended up deciding on flying ,train and bus.
Flight £87 return
Train from Airport to Totton £9.4 return
Bus - Totton to Dibden not sure how much this will be but should not be much.
Accommodation offered by CTC is 54 quid per night and breakfast
Total £205-230
As you mentioned you might be renting a car at £67/day which I assume you will have the car for 2 days at £134 plus the accommodation of £108 .Not even topped up fuel for the trip !!!
Just my opinion though ,go with what is comfortable for you
