It is unlikely that you can negotiate for a senior position, however negotiating for grade is different to negotiating for salary. It is possible to be appointed as a lecturer at salary points higher than the advertised ones and these can be used in the negotiations, particularly when hiring someone from overseas or from industry where salary levels are not consistent with UK ones.
A lecturer position is in a "Band" (actually band 8) and those bands have salary points and there is overlap with the upper "points" of the lecturer with the lower "points" of the senior (a band 9).
Being senior is not just about money, it is also about duties and responsibilities and they have a vacancy for the duties and responsibilities for a lecturer, and that is why it has been advertised as such.
Are your negotiating for different duties or more money? You need to be clear on that.
A useful references are:
- https://www.discoverphds.com/advice/after/lecturer-and-professor-salaries
- In which countries are academic salaries published?
- Within the UK, how are roles corresponding to academic grades defined?