Since the Government have changed the goal posts when it comes to the selling of petrol and Diesel vehicles, then there is no reason why Grand Union should not introduce a new service from London to Sterling using class 221 units with tilt enabled. As
@TT-ONR-NRN states a new train service being introduced is better than no train service being introduced.
Depending on who is in Government after next years general election and their policies, will be a case as to where any of the current Government Policies get changed. However, if they do get changed the changes I doubt will be immediate and may not be felt until 2026.
Whilst I understand the argument about having diesel under the rails, which is why Avanti went for using class 805/807 units on the WCML and EMR have done the same with the class 810 units in the MML, GU have to be operating the service at 125mph. Whilst you have got capable carriages in the form of the TPE MK5A carriages, you need a 125mph electric locomotive to haul them. Other than using class 221 Voyagers, the other option would have possibly been to use the TPE MK5A coaches if possible with a class 67 loco, but even though the class 67 loco is a 125mph diesel, I think that it is limited to 100 - 110mph when being used on passenger services, so this option would not have been able to provide the service that GU requires.
With regards the class 379 and class 350/2, you would have to spend vast more money on refurbishing these units to enable them to work the route for GU's requirement and the class 379 units would have to have their speed increased to be able to be used at 110mph, which as been said a few times is not cheap to be able to do.