The FL rules say that "Impact of purchase, sale and depreciation of fixed assets excluding players" is an excluded cost. Rent is charged on rented assets in lieu of depreciation on owned ones and so would be allowable IMO, I'm pretty sure the club will have agreed this with the FL.
Hmmm. I don't want to turn this thread into a technical B/S thread, so will make only one point then stand down on the point. The club don't own the stadium Fixed Asset, so, in the club's accounts, isn't the "impact of purchase, sale and depreciation" of the stadium zero? Meaning that rent is simply an operating cost which would not be excluded for FFP? This interpretation would also explain why the rent charged is as low as it is.
I do note that you say that you're pretty sure the club will have agreed the treatment of the rent with the FL.
I'll leave it there.