The Isles of Scilly absolutely win, hands-down, for me.
If we are counting the Channel Islands as being in the UK, they are also gorgeous, and with superb weather, relatively-speaking. The smallest inhabited island of Herm is a personal favourite, and Alderney, the only one I've not visited, looks stunning.
St. Ives, Carbis Bay, Padstow and Carlyon Bay in Cornwall are all very beautiful.
The South Hams in Devon are awash with lovely beaches: Bigbury-on-Sea is a personal favourite in this area, as the small, tidal Burgh Island is accessible by foot or sea tractor.
Lynmouth, nicknamed 'Little Switzerland', in North Devon, is gorgeous and unique, and Clovelly, with its very steep, cobbled street leading to the harbour, lined with old cottages, is worth the entrance fee you have to pay.
Another place I like, which is not so well known, is the 'tied' Isle of Portland near Weymouth, Dorset, and joined to the mainland by the long, sweeping Chesil Beach to the mainland, so not truly an island, but with an insular feel. It's more rugged than classically beautiful, but has a real appeal, and is very quirky; perhaps too much so, for some people. Attractions include a disused quarry, a castle, an abandoned railway line used by quarry workers, the large harbour used for the 2012 Olympic sailing, and the famous lighthouse, Portland Bill, at the tip of the island, exposed to crashing waves.
In keeping with its general weirdness, you get stickers in cars, reading 'Keep Portland Weird'. It has an 'end of the world' feel, and an eerie silence in many parts. Oh, and some of the older islanders won't say the word 'rabbit', as rabbits were associated with landslides which caused fatal accidents at the quarries; it's a superstition that still lives on.
True story.....