https://www.slab.org.uk/faqs/how-much-will-it-cost/
If it will cost us less than £100 to respond to your request, we will not charge you anything.
You will have to pay us any estimated charge before we send you the information. If getting the information costs less than we estimated, we will give you back the difference.
If we ask you to pay, we are not obliged to send you the information within 20 days of receiving your request. We can add on the number of days between when we ask you to pay and when we receive the money from you.