There is nothing like a few bad investments to squeeze those tier one bank capital ratios.
Crudely speaking, bank capital is the difference between assets and liabilities. Banks need capital to absorb any losses on their assets (which are basically loans) in order to cover their liabilities (which are deposits).
Tier 1 capital is high quality capital - consisting of shareholders' equity, preferred stock and retained earnings. It is the key capital ratio targeted by regulators.
In a properly regulated banking system, banks are required to keep sufficient capital in order to meet all losses, even when the economy slides into recession. The depressing reality of the UK banking system is that tier one capital was declining during the good times, particularly after 2000. The FSA, with their notorious light touch, allowed banks to expand without supplementing this growth with additional capital.
There is another depressing observation; since the credit crisis began, the weakest banks in the system have become a lot weaker, with ratios declining well beyond what is safe and prudent. From a regulatory perspective, many UK banks are insolvent.
However, we didn't need a chart to tell us that; the UK financial system's overexposure to the crashing housing market told us that months ago.