I am finding very good rates at www.baa.co.uk from LHR compared to booking from the US websites. About 30-40% cheaper even using the best codes.

However, these rates have a significant deductible of GBP 600 ("excess"). Visa/MC tells you to decline CDW but in this case I cannot. Will they cover the deductible?

Once, I had an insurance rental (after my car was hit while parked) from Enterprise and sure enough the rental is hit on the first night. Visa covered the $500 deductible on the rental car while my private car insurance covered the rest. The difference here was that the primary insurance was not CDW.