Dorset
Bugbog's best British beaches are mainly found in Dorset, Devon and Cornwall counties, in the south-west of England, since sand is abundant and the weather and waters tend to be warmer there due to the latitude.
Dorset is also favoured by the prevailing southwesterly winds bouncing over chalk cliffs to create a drier micro-climate stretching from Highcliffe, through Bournemouth and along Dorset's 95 miles of UNESCO World Heritage Jurassic Coast as far as Devon. In addition this year's official water quality guide gives Dorset a 100% pass whereas Cornwall and Devon have a handful of failures.

The curve of Bournemouth beaches and promenade extends from Hengistbury head as far as Poole's Sandbanks. More Bournemouth beaches pictures.
Crossing the narrow Poole Harbour entrance by ferry gets you to the magnificent 3 mile stretch of wild sand known by various names depending on where you live, how old you are and who you know, but Studland beaches seems to be the most common general term.
Continuing around the Dorset coastline and heading into West Dorset's 'Jurassic Coast', calm and characterful beaches keep popping up - along with fossils, all backed by the little-developed rural hinterland of the Purbeck Hills.
After Swanage town beach there are no sandy beaches until Weymouth but Lulworth Cove is attractive and bathable if you don't mind a little seaweed in your hair, with excellent walks around, including the Coast Path and a 40 minute walk over the hill to even more striking Durdle Door.

Lulworth Cove, a shadow of its former self.

Lulworth's Stair Hole, adjacent to the cove, and the path over the hill to Durdle Door that doubles as the SW Coast Path.

Durdle Door, gorgeous (especially when it's not garlanded with seaweed) but shelving a bit steeply for easy swimming. Try next door at...

...Man Of War (aka Man O' War) beach, sharing a ridge with Durdle Door and offering a light shingle (small stones) beach but with more protected waters than its famous neighbour; a popular swimming destination if you or your kids can handle the chill.
Other popular beaches along this coast, some rocky, some pebbly but all with peace, quiet, walks and views to die for (though few facilities if any) are Kimmeridge, Winspit and Ringstead Bay.

Chesil Beach and Portland Harbour seen from Portland, Chesil is the biggest and most impressive stretch of beach in the UK at 19 miles long, and the oldest at 6,000 years (give or take a few centuries) but...

...Chesil is not recommended for traditional beach lovers. More Chesil pictures.
Chesil is a barrier/storm beach, with pebbles (larger near to Portland and positively petite near 16thC Abbotsbury village and its famous swannery in the east) for seating or awkward walking, a steeply shelving slope to the sea, erratic winds, brutal waves and unsafe undertow current. Nor are the slippery, sinking stones good for a walk. However, Chesil does a good job protecting lowland Weymouth and Fleet lagoon from the sea. But 10 minutes away by car is...
...the next serious stretch of kid-friendly seaside at Weymouth, a lively tourist town, with shingle at one end of the long strand, manicured sand at the other, clear waters, a wide promenade, some attractive Victorian buildings and plenty of traditional British beach activities.

Weymouth, Dorset. More Weymouth pictures.
Finally, for those who like a bit of a treasure hunt with their beach-going, in West Dorset the beaches of Church Cliffs, Seatown, Thorncombe, Beacon and Charmouth are the heart of the Jurassic Coast, an area loaded with fossils (especially ammonites) going back 185 million years; walking tours are organised by the Heritage Coast Centre.
Nearby, pretty little Lyme Regis is the last town before Devon, in addition to being the UK's fossil capital.

Fossilized Lyme Regis, Dorset, UK. More Lyme Regis pictures.
Nudist Beaches: It is not illegal to strip off and do the beach thing in the nude anywhere in Britain. The problem is if someone complains...However, here are a couple of great beaches in Dorset where the nude life is either unofficially commonplace or officially permitted.
Official:
Studland beach, a large and sandy nature reserve beach with many grassy dunes. Just across Poole Harbour entrance from Sandbanks via the ferry.
Unofficial:
Hengistbury Head, below the rock outcrop.
Cogden Beach.
UK Water Quality: The Marine Conservation Society has published a Good Beach Guide based on results from last summer. Britain has less recommended beaches this year, probably due to contamination as a result of heavy rains washing sewage, petrochemicals and agricultural products into the sea. MCS suggests that you choose to bathe on beaches with a good water quality record but also avoid ALL beaches within 24 hours of heavy storms. Conveniently you can also download a free satellite navigation software plug-in.
High Water Quality beaches this year:
Dorset: Practically all beaches from Southbourne west to Charmouth are highly recommended.
Cornwall | Devon | Bournemouth | Studland | Sandbanks | Weymouth | Other UK beaches |