I wouldn't do liners *and* heavy duty tyres - that would be overkill. I suggest the Marathon Pros or Specialized Armadillo Elites. I haven't used Marathon Pros (although I note that the hire bikes in London do) - I have used Armadillos for years - almost no flats, but admittedly a terrible ride, very harsh.

I've had no success with slimed tubes, but tubeless tyres with sealant work really well. They are a bit of a pest to set up initially though.