They'll be charged for a 70+seater anyway, so no point keeping a -42, but unless they really push fares to pull traffic off the LGW and LTN routes, then they'll struggle to make much profit.
As the press release also mentions "slots"as one of the problems that had to be solved, I guess that they had to put an ATR72 on the route as allegedly LCY does not offer new slots for anything up to 50 seats (which leaves the question how Sun Air recently got their slots for LCY-BLL - the only explanation may be that they got them from the BA slot pool) - unless Aer Arann (or the IOM) bought the slots at LCY from someone else.