Well, spending well over £5 billion on GWEP increased the national debt by that amount. With 10y gilt rates over 5%, taxpayers are currently paying around £300 million a year just to service that debt. That annual cost is much more than the annual cost savings being realised by extending the OHLE beyond, say, Maindenhead.
Paddington was already electrified before GWEP. Class 800 and 802 are both bimodes and would be on electric in London.
We could have ordered high capacity bimodes to run the class 387 services, at relatively little extra cost. Either way, Paddington wouldn't have had diesel fumes. The extra costs today from running bimodes would be a tiny fraction of £300 million a year.
It was the project owner's responsibility to understand what was lurking under there. That was the railway. Pretending otherwise just shows how careless the railway is with public money.
That's all futures, though, isn't it? We were talking about benefits actually realised, specifically addressing this point:
Quite.
Getting back on topic, if the railway industry as a whole cannot bring a new service into operation after all the capital investment has been spent and the trains are ready to go, then it does not deserve public subsidy.