The 20th is of course the date the BA/bmi deal is due to be completed, BA only have a minority stake in Flybe (15%) so they could fly some routes under contract, it is no different from bmi using regional and the Dash has more seats and lower DOC than an ERJ, it will clog up the arrivals/departure flow though.
If regional is to go to IAG then i suspect there will need to be a revision of the DLH/IAG deal,i believe that a good number of the ERJ are owned, unless some are staying with DLH to feed its LHR operations from the regions until any time restriction from the EU deal have lapsed and they can be moved to IAG.
Perhaps we will see the Dash into here (BHD) on behalf of bmi/BA for this Summer, would make a lot of sense.