What speed of wind gust will cause a cyclist to swerve by 1m or more?
Having ridden near Bristol this morning in a similar forecast gust speed, I'd say you're right to be worried, but keeping aware of the wind direction and speed can make a huge difference.  For example: passing side roads on your left, if the wind is coming from the left, you may want to aim slightly towards them so that you get blown back towards the middle of your lane (swap left/right for US readers).  

Considering my ride this morning: I was lining up for a left turn at a T, then was suddenly closer to a right turn position as the gust coincided with me leaving the wind shadow of a house.  That was probably a sudden smack of 45mph wind broadside on, I'm tall and ride a hybrid with a rear pannier, and tend to sit up tall approaching that junction to see better (also loose clothing, so close to worst case).  It probably moved me 1-1.5m sideways.  I was already braking and unclipped.

If you're already riding in the middle of the lane you're in a good position.  But don't forget that when that wind is in your face you'll be doing something like walking pace, so you have to balance your right to the road with the behaviour of drivers, and that balance is shifted both because of your reduced speed and the worse-than-normal effects of close passes.  Taking the lane, but not slightly left of centre would make a big difference here.

Also:


Use your ears - you can normally hear a gust coming before it hits you.
Remember that when around queued vehicles the wind may change unexpectedly and you may lose most of your headway.
Don't forget that you will be going more slowly than normal, giving you more time to react (some of that wind will be in your face).
Don't fight to hold a perfect line - if drivers can see you being blown about they'll give you more room (I'm not saying exagerrate it either).