[OJFord]

I haven't submitted one personally, but from knowing someone who has (and has received them) and reading Private Eye (satirical & investigative magazine here) I gather a more common/easily usable excuse this side of the pond is 'commercial sensitivity'.

[rusk]

and heavy redaction

I’ve also seen fees being used which is technically legal and is supposed to be a reasonable administrative charge but can be multiplied by the number of subjects involved for instance.