Mixed Fleet Flying in Britannia/T-fly is like BB said, the possibility to be flying the 75 and 73 or the 76 and the 73. Note, not the 73, 75 and 76. This is being trialled as we speak and the company is about to extend it to more pilot for further trial in conjunction with the CAA.
Also with 640 pilots total and 350 or so on the 75/76 (nothing to do with MFF anymore), it is higly likely that many of these guys who aren't already Captain on that type, will bid to get just that.
Hence if you join today on the 73 as a DEC, unless MFF really happens, it will be a long while before you get a command on the 75/76 as suitability AND seniority will come into play.
WRT previous question, starting salary is 40Ks basic. Agreed average days off per week is 2.6 but depending with seasonal requirements this can be more. it varies so but I would say something like 4/5 ON, 3/4 Off is not unusual. Like I said it varies and pattern aren't fixed.