When I said 'carry-on only' I was meaning that AFAIUI she has booked on a low cost carrier with no checked luggage and would get stung if she, and son, had to check bags in both ways. She also wants to avoid dragging unnecessary luggage around. I suspect the ideal would be a cheap airport hotel she could use the night before her LHR flight that would let her leave her bags a few days before and pick them up for her stay.