Average LTV is always lower for BTL and self cert as its higher risk business and therefore lenders require higher deposit. Just because current default levels are low doesn't mean self cert and BTL are low risk. They are relatively new market categories and nobody knows where the top of the default cycle will be.
Re the credit squeeze, I think you have misintepreted my point. I said it had nothing to do with credit risk, which is the risk of their (NR's) mortgage assets defaults. You have confirmed that by stating that they currently have low default levels!!
Liquidity risk is precisely Northern Rocks problems as they rely on regularly packaging and securitising their mortgages and keeping a light balance sheet. The securitisation market has dried up and therefore NR have run into problems as they don't have a strong enough balance sheet to fund internally and have no access to the external funding market.
This is firmly a liquidity risk issue (which was driven out of credit risk issues in the US sub-prime market)