If you are asked for your salary expectations in an interview, then this could be difficult. If the job description has a salary range on it, then use that as a guide. You do not want to undersell yourself, but also not be too expensive. This can therefore be difficult and even more so if there is no salary range as a guide. It could be wise to find out what the going rate is for that sort of job and then state that you know the going rate is £xx and so you were hoping for something around that level.
If you have a figure in mind and are not prepared to work for less, then you may as well be honest.
If you are asked after you have been offered the job, then you are in a better position to negotiate and try for something higher.