"e.g. ancestor who was "adopted" by canadian family tree builders"
Always, always, treat any trees you come across on any site with a huge pinch of salt. Basically, do not trust them unless there is documentary evidence to back them up.
If you cannot find the information on MyHeritage then you are likely going to have to invest in an Ancestry or FindMyPast subscription (I would suggest Ancestry over FindMyPast, although I do have both subscriptions).
If you really do want to trace ancestors who lived in the UK then these are the two main resources.
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"e.g. ancestor who was "adopted" by canadian family tree builders"
Having said all of the above about other peoples trees, it depends when this happened. This may well be true.
Between 1869 and 1932 over 100,000 children were sent from Britain to Canada to work as indentured servants. These were children that were in orphanages or children's homes and it was thought that they would have a better life in Canada (also Australia and New Zealand) rather than in the slums of the UK.
Boys were trained in basic farming practice and girls were taught basic needlework and cookery. They were then shipped out at around the age of 12 or 13 to work as indentured servants for families in rural Canada.
It may well be that this ancestor was one of those "Home Children".
Attached is a photo of a group of children from one of the Barnado's Homes on their way to Canada in 1907.
Think of it as something like "Anne of Green Gables" but they didn't get to go to school.