Agree with Token Bird and P_S. London Guildhall are a good outfit and definitely not one to go broke, they're a massive great university and aviation is only a tiny part of what they do.
Bristol are the best ground school in the country and I can't see them going broke, if only because of their reputation and the loyalty they attract here on PPRuNe.
I can't say anything about OATS or Cabair because I haven't seen their material but I would be very surprised if either of them were to vanish off the scene in the near future - after all, the airlines are still sending their cadets there, I presume that sponsoring airlines have access to the flying school accounts and would run a mile if they looked at all dodgey?