yields seem quite reasonable - several showing as £100+ o/w.
Don't want to be too picky, but £100+ is the marginal price for the next passenger, not the yield, which will be the average of the prices of all the tickets sold (including any cheapies that are already sold, so as of now, it's a safe bet that the yield is well less than £100.) I agree that if they are actually getting people booking now at £100+ one-way, that appears a reasonable sign, but it also depends on how clued-in their revenue management is.
However I do agree most heartily with you, sir, that in evaluating route performance from the outside there's no subsitute for actual £££-based evidence!