Say what you will about the big accounting firms, no partner or member of staff or their related parties is allowed to own shares in any worldwide audit client of that firm. They have very strict disclosure and compliance rules around this. There are numerous stories about graduates at these firms having their careers ended by buying shares of companies which are audit clients in some other office than the one they work in and being summarily dismissed.
No EY employee worldwide would be allowed to own BLA shares for independence reasons, which is why the sub-leasing of office space to them by an audit client raises such eyebrows. Even if it is on normal arms length commercial terms, it looks questionable from an independence perspective, which is what the accounting firm should be scrupulously protecting.