Edit: it turns out I was neither removed nor required to leave. No error was made.
I made an error on my last application for entry clearance while answering whether or not I have ever been removed from another country. I answered no--my 10 year visitor application was approved.
However, now, three years from the date of issue of the entry clearance I have realized that I was in fact "removed" from Canada about 8 months earlier. I used to be a Canadian Permanent resident but on entry, the IO found that I did not meet my residency requirements. I was allowed to enter but given a "departure order" to leave within 30 days. I did leave and have never needed to enter Canada again.
So now what? I still have seven years of entry clearance remaining. I did not mean to deceive anyone. I just had just assumed that a departure order was issued because my PR status had been revoked--and it wasn't a "removal" per se--just a 30 day leave to enter. The removal order would not have been material to my UK application IMO. In fact, it would prove that ties to my home country are so strong I decided not to move to Canada legally. Do I do anything now? How do I proceed on my next entry clearance (which is very far away of course)?
I know Canada does not share immigration details about its citizens and PRs (which I was when I was "removed"). But I am no longer a permanent resident. Any suggestions for which lawyers I should contact?