I drove from Swindon to Chippenham via the M4 today in the carat around 11am. Overtook a lorry and then got blown over to the outside lane! Very scary. Thank god there was nothing in that lane.
If you're going out on the bike, I'd say avoid the motorway and be very careful on open roads with sidewinds. If the wind can move a heavy VW Golf then I daresay it can move a bike.