I can't do much harm to try to negotiate - deciding to appoint someone to a lectureship is a difficult decision,and if they have picked you, it is unlikely they will change their mind if you try to negotiate better conditions.
Particularly the Lecturer/Senior Lecturer split is general competency based, rather that need based - that is people are promoted to senior lecturer when they meet a certain set of performance criteria, rather than when they need someone to do an SL job.
So, at my institution, to be promoted you must demonstrate you are doing the job of a Senior Lecturer, and that is defined by meeting 6 of the following 10 criteria:
- Outputs: At least 1 output regarded as "internationally leading" (think a paper top journal in your field, or a generalest glam-journal) in every 2 year rolling period.
- Income: Average income over a multi year period that at least matches that of the average senior lectuer in a Russel Group university for your field.
- Impact: Deliver research that forms a viable Research Excellence Impact case study, or potential future impact case study - patents, change government policy make an ecconomic or social difference.
- High Quality Teaching Practice: Evidence that your teaching is good- outcome data, teaching evaluations etc.
- Curriculum enhancement: Review and redesign an existing complete program of study, or design a new one (or a significant number of indevidual modules).
- Improving teaching practice: Evidence of pedagogical scholarship, research and publication.
- Academic Citizenship: SL level admin duties like admisions tutor, director of Equality, Diversity and Inclusion, chair of the academic misconduct committee, etc
- High Quality Management: Not just undertaking admin for the department, but that you are managing others to achieve the aims of the department, faculty and univeristy.
- Change and Innovation: Make admin stucutures or functions better.
Professional Standing and wider engagement: Basically show that your field (or the public) knows who you are - international conference invites, journal editorships, external examining, etc.
I supsect that if you can show that you meet 6 of these (and are prepared to continue with such duties), or whatever the equivalent criteria are at the univerity that is hiring you have, then you could argue that you should go in at SL level. The worst that can happen is they can say no.
Interestingly we pretty much never advertise for lecture level only, we generally advertise Lecturer or Senior Lecturer.