The article is a little confusing. Who is investigating the actual case? England? Russia? Both? Are they cooperating?
The charges are that this guy and some others had bad times and took some $500 million loan in bank and used stocks of fake company as a collateral and also forged documents which said the stocks worth $500 million. Then they defaulted on loan and left the bank with collateral.
In fact Calvey has an excellent reputation in Russian business circles as a honest and principled man so it came as a surprise to many.