There was no way they could have operated the proposed schedule using only one aircraft. Four rotations per day to the UK could not have worked due to the restricted opening hours of Sion airport. That's before even considering the need for some schedule padding to take account of the various minor delays that typically may occur in day to day airline operations. And of course the weather and minima factor.
Plus, with a u/s aircraft, what would be their chances of obtaining a short notice substitute with a Sion qualified crew ? Quite likely any replacement would have to use Bern,. Basel or Geneva.