Monarch Scheduled & Excel Airways
Have flown many times with Monarch Scheduled (from Manchester) and always been totally satisfied. They have a good product at reasonable prices. Specific seats can be booked in advance at a supplement and a good range of food is available for purchase on board. Good record of punctuality, stress free check-in and friendly & efficient cabin crews.
Recently flew with Excel Airways and could not fault them in any way. On flights suffixed with a 'v', if you require a meal on board it must be ordered and payed for at the time of booking otherwise only light refreshments are available.
Would have no hesitation in recommending either of these airlines but in your case with an elderly relative in the party, the booking of seats of your choice in advance, perhaps Monarch Scheduled may have the edge providing of course that price and flight timings are similar.
In my opinion, there is no chance whatsoever of either of these two reputable airlines cancelling a flight and leaving you in the lurch.