Firstly, Wickes will not undertake just tiling. These rates are part of the extras work rates to go with a full bathroom installation. The installer Wickes allocate to do the job will do the whole lot - job done - without you waiting for separate trades to do each part.
Wickes are competitive when up against other major diy chains but you will always find someone to do the project, any project, cheaper.
They are not in it to compete with your local tiler, plumber etc. There are also higher admin charges over and above a small company. Wickes customers like the fact that the job is taken care of by one organisation, which, btw, covers the entire job with a 2 year warranty following completion.