I'd expect to do Brighton a fair bit quicker tbh. Unusual for that drive down to take more than 1.5 hours, and Bath in anything under 2 is a result. The A36 isn't the greatest road in the world, either.
Tis the wife's favourite Big Day Out, (it is a lot nearer for us, than you Brighton-based folk). There's not really much to DO tbh, so I'm surprised that kids would be that on board. But yes, if you enjoy strolling about, looking at pretty buildings and pretty people, or sitting in the sun...