Catalysis of the reaction: 5-amino-5-(4-hydroxybenzyl)-6-(D-ribitylimino)-5,6-dihydrouracil + S-adenosyl-L-methionine = 5'-deoxyadenosine + 7,8-didemethyl-8-hydroxy-5-deazariboflavin + H+ + L-methionine + NH4+.
FO synthase