Great Western Coffee Shop

The £15 singles offer is back (under the SWR banner). Saturdays, Sundays & Bank Holidays until the end of June (travel by 1st July, book the day before minimum). There's a whole raft of other restrictions and the list of eligible stations looks distinctly more limited than before and nonsensical in places (e.g Clapham Junction to Waterloo for £15, no thanks):
https://www.southwesternrailway.com/destinations-and-offers/offers/summer-weekends

Here's also the detail of the Summer Saturdays fares for Weymouth and Corfe Castle mentioned in another thread:
https://www.southwesternrailway.com/destinations-and-offers/offers/summer-saturdays

Travelling into London:
From any of the following stations on the South Western Railway network: Weymouth, Upwey, Dorchester South, Moreton, Wool, Wareham, Holton Heath, Hamworthy, Poole, Parkstone, Branksome, Bournemouth, Pokesdown for Boscombe, Christchurch, Hinton Admiral, New Milton, Sway, Romsey, Mottisfont & Dunbridge, Dean, Salisbury, Warminster, Tisbury, Gillingham, Templecombe, Sherborne, Yeovil Pen Mill, Yeovil junction , Castle Cary, Bruton, Frome, Westbury, Trowbridge, Bradford on Avon, Crewkerne to London zones 1-6 stations: Barnes, Barnes Bridge, Berrylands, Brentford, Chessington North, Chessington South, Chiswick, Clapham Junction, Earlsfield, Ewell West, Feltham, Fulwell, Hampton, Hampton Court, Hampton Wick, Hounslow, Isleworth, Kew Bridge, Kingston-Upon-Thames, Malden Manor, Mortlake, Motspur Park, New Malden, Norbiton, North Sheen, Putney, Queenstown Road, Raynes Park, Richmond, St. Margarets, Stoneleigh, Strawberry Hill, Surbiton, Syon Lane, Teddington, Thames Ditton, Tolworth, Twickenham, Vauxhall, Wandsworth Town, Waterloo, Whitton, Wimbledon, Worcester Park