I assume the £1.8Bn could include costs for delays - interest on capital etc. so could be including various issues driven by the conversion decision.
It would be interesting to know how much of the F35B and F35C cost goes into the UK defence industry - and hence is also recouped as tax (if any). I wonder for example if the RR workshare in the lift fan more than compensates for the extra cost. What level of tax is typically paid on US acquisitions of this type?
PS orca - google 'crpa' for one well established jamming solution