I got crushed oystershell from a supplier called “Charnwood Milling”. They posted me 2 x 25kg bags. Postage cost as much as the shell but it still only worked out at £20 for the 50 kg. Much cheaper than coral gravel I’d have thought.
Having said that, can’t see any reason the gravel would not work.
Can get you a phone no. for Charnwood if you want.