Welcome to hell. This advisory hasn't been the thing that takes your car off the road, but it's telling you something extremely important - you need to act now and I bet if you get under there with a toffee hammer and an honest whack, you're going to find out that she... needs some honest work.
So, I know you know your cars, so I'll say this for anyone else watching: I didn't take that corrosion seriously enough in time because the warning wasn't strong enough. Your MOT guy doesn't want to take your car away from you, but he's calling out your chassis because if you get T-boned in a decrepit car, you're dead.