I'm not convinced by arguments based on fuel freezing - if the TAT was -37 and the fuel freeze point -57 how did it freeze? The freeze point is the appearance of crystals, not the point at which the fuel goes solid, and the margin was 20°C, not the 3°C required.
Indeed, but how about the fuel temperature in the pipework? could it, somewhere down the line, get below the temp measured in the coldest spot of the tank? could part of the fuel, during the descent towards LHR, when idling, begin to freeze and cause an (temporary) obstructon?