See http://www.lightningmaps.org/?lang=en . So far the storms are far away, so I can remain on air for hours still. There is an outside chance of storms around early evening.
UPDATE 1220z: The first lightning strikes have happened - in the North Sea just off East Lincolnshire. I shall keep a close watch.
UPDATE 1445z: At the moment, the only storms are over the sea off eastern Kent. Staying on the air for now.
UPDATE 1700z: The nearest storms are in France and Belgium. I think it unlikely I shall have to go QRT early tonight.