If they don't they will be forced be authorities in UK/Europe anyway at some stage.
Legally now impossible to enforce as the takeover has proven the basis for decision was incorrect. Attempts by a majority shareholder to enforce a sale would set all kind of precedents that UK would want to avoid.
Lots of legal precedence and protection for minority shareholders and UK would basically have to scrap them all.
It would basically mean that takeover law would change which would annoy Tory paymasters.
In practice Ryanair will probably just take the cash after forcing this to go through monoplies and mergers review.