I guess the issue is they want to pay PL pilots a lot less than mainline pilots and is this something that couldn't be done with PL pilots on the main list?
Not so. BA used to pay pilots in the regions and LGW less than those at LHR and they were all on the master seniority list. BALPA have already told BA they are willing to accept different terms and conditions at Lauren than within BA in the UK, all they insist upon is the pilots being on the seniority list. One must wonder what BA's real intentions are if they are so resistant to that.
In fact, having just looked at the proposed pay scales, it would actually be
cheaper for BA to pay Lauren pilots the appropriate mainline pay for their first 6 years! If BA want to keep costs down they could simply cap the pay at that point and the Lauren pilots can go to mainline with 6 years seniority or leave. Somehow I doubt many people will be intent on staying beyond 6 years anyway given the debilitating nature of the Lauren work.