UK Lecturer application: possible to refuse to answer "expected salary"?
(This does not really answer the question of whether one can refuse to answer "expected salary": rather it is a suggestion to help answer that question in a reasonable way, if you choose to do so.)

Something that may be useful to be aware of is the "Higher Education Pay Spine" which most (many? some?) UK institutions adhere to:

https://www.ucu.org.uk/he_singlepayspine

In my (admittedly limited) experience, Lecturers are typically appointed at a point somewhere around 36 on this scale. Others may have different information: please say if you do!

(For particular universities, a bit of internet searching will often show that they use a different scale, but the one linked above gives the general idea.) 

Depending on your level of experience, you may wish to adjust what you ask for, but at least this gives you some context to help calibrate your response.