What is a 'two tab third pilot'? We employ first officers, not skivvies. When you go on line in this company, you are fully qualified on your aircraft type, and will operate on two-man, three-man, or four-man crews as required by your roster. You will operate as P1u/s (PF) from day one - when it's your turn. I repeat: we do not employ second officers or cruise pilots.
What do you mean by 'who they use for type ratings'? We do our own training in-house.
I think your sojurn in the USA has given you a US-centric idea of how airlines work. The UK is quite different from the USA in many ways.