https://www.slab.org.uk/guidance/situations-in-which-retrospective-authority-for-an-increase-may-be-granted/
Usually, you need our prior approval to exceed the cost limit. We may grant an increase in the limit retrospectively if we are satisfied that the additional funding would have been agreed and that there is a special reason why prior approval was not asked for. This is more than simply forgetting to ask.
Suitable reasons may include:
If you apply for exceptional case status or an additional fee post- case, we will be prepared to consider a request for a retrospective increase in case costs if needed. This happens once your costs have been calculated on a detailed basis.