First of all, the DDA requirements mean that the train has to be accessible for the disabled, not every single carriage on it. For example it would be perfectly legal to fit one Universal Access Toilet in one carriage only. There are brand new EMUs entering service with one UAT per 4 or 5 car set.
Then the power doors only need to be at one position in one carriage only, not every single door position.
And the DfT has power to make derogations from the rules anyway.
In threads covering other HST operators (and future operators such as Scotrail) it has been said that there is a simplified approved upgrade, which involves modifying all the doors to make it possible to open them from inside without lowering the droplight, once the guard has operated the door release, and fitting power doors at a limited number of positions. There is already a retrofit power door design for Mk3 bodyshells in service with Chiltern.
The number of carriages needing anything other than the simplified door mod is probably only two per set (one for First and one for Standard maybe), and EMT have 12 sets, so 24 carriages. This is probably cheaper than rewiring all 24 HST power cars to supply Mk4 stock, or modifying 12 complete Mk4 sets to accept the power feed from existing HST powercars.
No, the power doors have to be fitted across the train. The accessibility regulations take a much, much wider view of impairment than might be obvious, which make it impossible to fulfil them by providing only a small 'accessible' area in one area of the train. For instance, the requirement for contrasting handrails and lighting in the door footwells is to serve people who may have only limited sight, but who can otherwise use the train as an unimpaired person could. It's often not possible to determine who these people are ahead of time, so it's not possible to restrict them to only using one part of the train.
The power door sets can still have swing doors but only for staff use. These are doors which aren't even painted in contrasting colours and are often located in the galley. Both Chiltern and ScotRail sets do this, as will any other conversions in future, as there is no need to change a door which can't be used by members of the public.
The cost of 'Chilternising' a Mk3 carriage appears to have gone down compared to the initial Chiltern effort, so it's now much more likely that these conversions will go ahead even if they wouldn't then be in use until 2030. Since the passenger saloons already have all the mod-cons like WiFi and power sockets really it's just a matter of the CET toilets and the doors. The plug door design that will be used by ScotRail is much simpler to implement than the Chiltern design, as the new door can basically fit into the existing aperture rather than needing the whole end of the carriage to be replaced.
While the Mk4 carriages may already be closer to being fully compliant, the costs of running them are somewhat higher and they're not already cleared for the vast majority of the GB rail network. If the cost of running Mk4s (and doing whatever conversion is necessary for the ETS supply from HST power cars) is higher than the cost of running Mk3s and doing the plug door work, then the Mk4s will end up being scrapped before their predecessors.
