SEN is different. Firstly it serves London. It has a large catchment area which is highly populated and affluent. BOH, whilst wealthy, has a fairly small catchment - with half of its catchment area being the English Channel. To the west is BRS with over 50 destinations, to the East LGW with 100+ destinations.
The fact that EZY have over 20 bases and only operate flights from one of them speaks volumes to me. There's no Milan, Berlin, Hamburg, Paris, Nice, Lyon, Barcelona, Toulouse, Venice or Naples. Nor are there any flights from their UK bases. They could easily operate those flights from BOH but have so far not seen the demand to warrant flights to them. Nor have they operated W patterns to anywhere else (except for GVA).