A rare sight: blossom in winter. I think this is Prunus x subhirtella which the Royal Horticultural Society lists as a naturally occurring hybrid from Japan. A cultivar named 'Autumnalis' is available from suppliers in the horticultural trade, one notes that it was introduced to the UK in 1894. Presumably that's what we're looking at here.

 Most cherries flower in spring but Prunus x subhirtella is aptly known as the Winter Flowering Cherry.