Just for completeness, here's the conclusion of my correspondance with TFL.

It has been suggested in news reports that the reason for the bus explosion was that Hasib Hussain wanted to get onto the Northern Line at King's Cross, but could not, because the service was suspended. Can you comment on that?

Answer - I would refer you to my earlier comment that train services were operating on all six lines through King's Cross.

Also I understand from people who were in the area at the time that Oval station was closed before the bombings: was the Northern line service starting at Oval or from Kennington?

Answer - Service was suspended between Morden and Stockwell prior to 08:50.

I suspect that if we had any proper journalists examining the story of the tube closures in detail, they would find that it isn't quite as simple as is being claimed. Many ordinary people think there must be a connection between the earlier closures and the bombs, and suspect a warning which is now being suppressed -- even if they otherwise fully accept the official account.

I've been told personally by people I trust that Oval was closed before the bombs and that the reason given at Morden changed from "broken down train with a broken axle at Balham" to "power surge" before the bombs.

There's another Israeli story mentioning a warning here:
http://web.israelinsider.com/Articles/Security/5997.htm.