Yes the 228 will be very expensive into LCY we looked at it back at Air Wales then used the 42 but cost even then where very high. As its a 42 then as BI have already the additional training and experience it makes sense.
As to the Tris nominally there are 5 in the fleet. 3 for daily use one each based ACI, GCI and JER with one spare at GCI and one on maintenance. With the new arrangement for GCI-JER that will reduce the fleet requirements hence two to three 228's only required.