I think Mk1s can be expected on the August steam tours, should be Mk2s for the rest.
With regards to traction, as pointed out it'll be diesels from the WCRC fleet. From what I hear it is unspecified so can be pretty much anything, but with 95mph capability and ETS provision I reckon most will features 47s (or Ronnie), but don't quote me if a 33 or a 37 drops on.
For those who fancy rare track for loco-hauled passenger trains these days, the Bank Holiday Monday Southport-Carlisle tour might be of interest to some as the last regular booked loco turns on the Southport-Wigan line was in 1994! Only charters or engineering trains have brought locos to the line since, and even then it is few and far between.