My standard Victoria plum has flowered first this year and is in full flower today the 28th March a whole month earlier than last year! I live in the Vale of Clwyd, Denbighshire, north east Wales.
The Opal, Jubileum, Denbigh plum and Ontario are all close to opening, Marjorie’s Seedling is about a fortnight off flowering.
Victoria plum in flower, though not full flower, at the National Fruit Collection Brogdale (above). Dates of flowering for Victoria plum last year were: 10% of flowers 10 April; full flower 14 April; so about two weeks in advance this year in Kent: see here