In theory, it shouldn't be an issue, since ECUs aren't that complicated to make and there are plenty of aftermarket manufacturers around who could do the job - Emerald and MBE are the obvious UK choices.
If the relevant systems can be reverse engineered so as to ensure compliance with relevant emissions controls, the Government aren't bothered by any replacement ECU.
The problems really seem to be stemming from manufacturers seeking to prevent owners from being able to repair their stuff - General Motors and John Deere, for example, are seeking to deny that purchasers of their vehicles actually own them as such, but rather they have "An implied licence to operate the vehicle", due to the extensive nature of the software that controls them. So you're not allowed to fix your car, or get someone else to fix it without their approval because it's not actually your car!
So what Rog down in Somerset is doing is very sensible indeed. Fortunately my ECU is mine, fitted to a car which didn't have one originally, but if you want to keep your car for a long time, it's probably worth thinking about how that might be achieved if/when your ECU goes Phut!
