Has there ever been a bad tornado in the UK?

Yes. The worst tornado to have hit the UK in modern times was a T5 (equivalent to a strong F2) which hit Birmingham in July of 2005, costing £40 million (about $70 million US). This is a rather significant tornado, even by US standards. The strongest known British tornado struck Portsmouth in 1810. It was estimated to have been a T8, equivalent to an F4.