If I understand right, the largest power failure in the UK (not counting the coal miner strikes) was the 1987 storm, and that didn't reach 3% in the UK*, more like 1.15%. I can't find exact figures for the percentage affected by the 2006 European blackout, but assuming the region boundaries and population hasn't shifted too much since then, that was also a bit less than 3%.
* nor did it reach 3% in France, which was affected worse; but a quick search didn't give me a list of French power failures to compare sizes of.