LONDON (Reuters) - Britain's services sector shrank in November to mark three months without growth for the main driver of the economy, a survey showed on Wednesday as uncertainty over Brexit and an imminent national election weighed.The IHS Markit/CIPS UK Services Purchasing Managers' Index (PMI) fell in November to 49.3 from October's 50.0.The reading was revised higher from a preliminary 48.6, but the index has still not exceeded the 50 threshold for growth since August - the longest such run..