A Schengen visa or residence card from a European country does not, by itself, authorise a tourist visit to the UK. UK visitor requirements apply separately and must be checked against the passport used for travel. Certain documents can be relevant to specific transit exceptions, but those exceptions are not equivalent to permission for a holiday. Whether you pass through border control, collect baggage or change airports may affect the transit assessment. Read GOV.UK guidance against the actual itinerary. An airline selling a ticket is not evidence that the passenger holds the required UK permission.