The airframe component of the cost is tiny compared to the electronics on something like an AWACS. The original 707 based USAF AWACS back in the late 1970s were going for ~$100 million, when you could buy a brand spanking new 707 for a bit over $10 million.
Furthermore, the biggest portion of the electronics is not the actual electronic equipment, it's the integration - making all the different systems interface correctly and play nice with each other. Heck, just basic stuff like ElectroMagnetic Interference (EMI) is a massive problem on something like an AWACS aircraft.
I don't know how much of the avionics will be changed to 'Anglicise', but if it's extensive, the integration will be horrendous and could easily turn into a trainwreck.
I suspect the poster who wrote that the UK will end up spending more for something that doesn't work as well is dead on.