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.

In my experience applying for, being hired for, and hiring for jobs in the UK the stated salary box matters a lot less than you're thinking. It isn't something that you are bound by at all, rather it gives them a sense of if you are serious about the position and are working within their possible range. They want to make sure they make an initial offer to you that you will consider, they aren't trying to bind you to a low ball figure.

Answer honestly for both what you make now and what you'd accept: expected salary should be what they would have to give you for you to say yes to the job. That number is negotiable later e.g. what if your current employer makes a counter offer to keep you and they want to counter that counter offer.

If, as you say, they state a salary range on the job advert. Simply put that in the 'expected salary' box. Better yet, put the top-end of their stated salary in the box.

This site recommends the same approach