I agree with the comment above that the OP needs to do a little more research.
I would also advise that applying directly to a TOC for a trainee driver role might be a better bet than becoming a CSA with LU, with no guarantee of ever getting a train operator role. TOCs offer more money than LU, most have a four day week (and lots of overtime if you want it) whereas LU is a five day week with no overtime. LU is largely ATO these days (depending on line) with some incredibly intensive work. LU operators can also find themselves sitting in a “pool” link, which is effectively permanent spare, with no visibility over their work.
As stated above an LU operator joining the mainline would be starting from scratch as a trainee, and would also take a large pay cut to train.
It’s by no means unknown for LU train operators to join the mainline, but I’ve never heard of anyone going the other way.