However as smoeone said above the Uk has regularly changed immigration rules so the fact it was utterly uncertain would be part of people's decision making process when they decided to risk it and move here but not become a british citizen and not to give up their original passport and put themselves in the same position as most people in the UK
There's a difference between changing rules and applying to new people and revoking what was given to them, that what a number of posters have tired to explain that some posters just can't get.
Imagine you are referred by your gp to the hospital for an op. You have it, you feel good afterwards and you didn't have to pay for it because you are told the nhs is free at entry. 10 years later, the government decides the nhs can't cope, people vote for a law couldn't vote for and/or agree to, and 1year later, they decide that all those people who had the particular surgery you had, amongst other treatment, need to pay a contribution of £65 towards that surgery to the nhs going back 25 years otherwise, you cant ever have access to the nhs again.
Then people tell you 'well you knew that the law could change at any time, it's known to do that, you shouldn't have had that non essential surgery at that time', and stop moaning, it's only £65, I had to pay £2000 to have that surgery done privately so count yourself lucky!