Say the air pressure is 1035 mbar for his attempt. A low pressure system over the UK could easily be 50 mbar lower than that, i.e. a 5% difference. Power lost to drag is linear with air pressure, so if this were a flat TT where virtually all power goes into overcoming drag, we'd expect to see a 5% difference, which at 50 kph means going 2.5 km slower. For an hour record in a velodrome, power lost will mostly be split between overcoming air resistance and accelerating himself round the corners - I don't know the split, but if it's a portion of a 2.5 km pie, 1 km sounds eminently feasible.