Are you seriously suggesting that WX can take on and beat FR in a price war?
(In the case of SNN-MAN, an FR 737 beats a Stobart ATR72 on seat cost any day. Somehow I think you'll find that the FR 737 beats anything Cityjet can put into the market too.)
No!
If FR lose traffic they will lower fares to increase loads and grown the market, if EI lose traffic it will open more transit traffic via LHR and some lower fares however EI have no choice but to stay put at current freq for 5 years....once IAG goes ahead.
FR/EI have had it easy their pricing is not cheap so some competition will bleed money off them regardless.
There is clear demand for LON especially on peak days where 100+ euro is charged by FR and they are filling those aircraft. A 2 daily (3 daily peak days) should work and deliver competition. If they get 100 a pop like FR are getting now then they will be doing well and that's a rough idea of what BA/WX are getting ex DUB-LCY.
As for the SNN comments, FR have grown the market by adding extra capacity and lowering fares. The same will happen here and I think WX can make a service viable.