Camber Sands is nice, Rye is a bit odd but has character.
Nice long sandy beach, dunes, sea, able to park (many car parks dotted along the north end of the dunes) just take food and water because it’s a bit of a hike back into Rye.
It’s another surf/windsurf location we use a lot over the year, has to be windy on a low incoming tide to work though.
The main problem with the beaches east of Bournemouth through Eastbourne is most of them are shingle. Whilst not actually a pain to play on (and plenty do) it’s just a bit, well, crunchy.
The main problem this time of year is everyone heads for the sandy beaches down here and there just aren’t that many, hence the crowds.
For London, why not head East? Essex and Suffolk have some exceptional beaches, around the Blackwater there are some secluded crackers and near small villages too (West Mersea for instance) or go south of the river into Kent and you’ve a whole host of beaches from Whitstable down to Dover, and beyond to Dungeness (although that ones shingle too)
A12 and A13 can be a PITA but the A2/M2 and A299 flow reasonably well even at peak times.