Quick question your journey time to Longleat i am guessing is a little over 2 hours,that is not a long journey,kent wouldn't be under 3 hours for you as well Blackpool would be over a 3 hour drive away for you!The only drive that I do that is over 3 hours away from me without stopping in the area to do other places is Colchester which is over a 4 hour drive one way,I have done Paignton in a day in the past which again is over a 4 hour drive in a day,never again as there are far too many collection in the area that can be done in that area.
Personally I would do what Johnny Morris has suggested and go to the furthest place you plan on doing and find somewhere to stop and do another place close by the next day,you never know you may find that you could visit 2 or 3 small places in a day and increase the number of places that you visit in the time period you have!!