War, The EU, And The Year 2007: Why UK And US Clocks Go Forward On Different Dates
But in 1996, the EU decided that too many differing time zones were inconvenient and expensive. So member countries, including, at the time, the UK, all took on the last Sunday in March as the date clocks went forward.
Article source: https://www.huffingtonpost.co.uk/entry/uk-vs-us-dates-clocks-go-forward_uk_69aa97e7e4b03ae2f885ed04