IO540
The MOT test was introduced to protect the public from vehicle owners who were not maintaining their vehicles correctly and keeping them roadworthy. It had nothing to do with protecting the public from cowboy car dealers.
I do not know why you are so hung up on the idea that the only reason that action might be taken against foreign aircraft based in the UK is revenue generation. The CAA are not a profit making private sector company, they are a statutory regulator. Their charges may only cover their costs. They have no incentive to generate revenue. However, if they were to take action it could be to the benefit of UK aircraft owners and licence holders. If it resulted in an increase in the number of UK licence holders, aircraft owners, etc, the CAA might have scope to lower individual charges.