In this instance I would say bump steer is road rather than a car related issue. Two months ago I drove on a similar road to the one he used to exemplify the issue:
Twisty b-roads
Negative camber with many bumps at the edge of the road.
At one point, the MX5 that was driven in front of me did a vigorous wiggle from left to right. I drove over the same point and could feel my steering wheel pulling left to right to left. Had the road been wet, I would likely have been in a ditch.
The 20min stretch of road was the most demanding one that I've ever driven on. My confidence in the car's ability to maintain a contact patch with the road was truly tested. My heart was racing once I reached my destination!
This is what roads in the south east of England can be like. They can make your car's suspension feel like crap. Out of the factory my former F25 X3 was tram lining like mad. Swapped for non-run flats and the rode exceptionally well at all speed.
Drive in Manchester or Scotland and a GT3 Touring will feel like an S-class, the roads up there are smooth and straighter for longer. Therefore I wouldn't rush to fettle with the camber on a GT3 in order to "fix" its behaviour on UK roads.
Below is an example of roads that I drive on when I go on a blast. Some include sunken manhole covers that will crack your alloys if your car is fitted with low profile tyres. My suspension, tyre and alloy choice are of a combination for the car to thrive on roads like these. The car needs to be comfortable with, at high speeds, dodging or driving over poor surfaces.
Similar to 911s and some sports cars, my M140i I fitted with progressive springs. They have two spring rates. In practical sense, the ride is smoother and more settled the faster I go. The car can be driven at 70-80mph on the road below with good composure. Zero regrets ditching adaptive suspension for superior passive setup.