The 1100th Anniversary of Alfred styling himself 'King of the Anglo Saxons' was widely marked in 1986, and therefore I think it's appropriate to begin planning to commemorate the 1100th anniversary of another of the most significant events in England's founding.

Whilst this is a suggestion for a one off bank holiday, I think the timing of this holiday is also preferable to another new English bank holiday that some propose, St George's Day, as St George's Day takes place on 23 April each year, close to a number of other existing bank holidays.