Or do what they should have done in the first place. Put the M11 through a tunnel and give DX the full original length of runway again.

Indeed.

This would still be the easier option……although still enormously costly, as you would have to dig out a bit of the M11 to enable the reqd clear height for ventilation, but a 100m or so of concrete box dropped into a graded out trough with the runway extended over the top is a relatively simple proceedure.