The 24 refered to is the production slots, the RAF will still receive the 89, but the first 24 Saudi aircraft will be made when the RAF aircraft would have been made, the RAF Aircraft then get placed into of the production run where they can (usually towards the end).
There is quite a bit of slack in the 4 national assembley lines, but there are problems with long lead items, if you want a quick delivery then swapping production slots for export is an excellent way delivering at short notice.
Given sufficent time (18 months) there can be a large increase in production per month to fulfil the biggest export orders.
Cheers