Barra and Solas on the Western Isles both have beach runways, but it's imperetive that you ring and get a briefing prior to landing - they're both PPR anyway - the tide comes in twice a day and at very high tide most of the runway is covered.
Loganair fly a DHC6 twin Otter from Glasgow to Barra every day, if you ring
Loganair Head Office, Tel: 0141 848 7594 and ask to speak to the chief pilot of the Twin Otter Fleet I am sure that any of the guys will be more than happy to help.