Hmmm, I wonder how long the drive from Dover to Cornwall would take in peak season? Days

As JonRA suggested, I would look at Suffolk and Norfolk – lovely beaches, mostly clean sea and good beer (if that's your thing). On the way back, stop for a couple of nights in Cambridge, then onto London for a few nights.

If you were intent on the South West, then Dorset and Devon are lovely, with clean beaches and sea, plus beautiful surrounding landscapes or cities if you want a change.

Cornwall is vastly overrated IMO and full of knobbers during summer.