I'm increasingly of the view that the long-term solution for rural routes that will never be electrified will be LHCS. It will always be easier to build in filters and so on for an engine in a loco with a lot more space than an under-floor engine in a DMU. In extremis, there is even the option of attaching a tender to the loco and routing exhaust through the tender.
These rural routes will never have high demand (by definition - if they do have high demand, then they would justify electrification), so the trains are not going to be long enough that a loco will create problems with platform lengths. The rural stations will be fine to have the loco hanging off the end of the platform if they have very short platforms (and SDO if needed); there won't be points to foul and signalling is pretty basic and easy to move to allow that. These lines tend to run to a more major station at one end (e.g. West Highland into Glasgow, Far North and Kyle of Lochalsh to Inverness), where there will be room for three or four carriages and a loco (and, probably, a DVT/DBSO to allow push-pull operation) on a platform that's probably also taking six or eight coach EMUs.
Obviously, LHCS is not as efficient as DMU operation, but these lines are getting a handful of daily trains, not a multiple tph service, so they're having to be subsidised to exist anyway; balancing cheaper purchase against higher running costs is a perfectly reasonable decision.
For the rest of the network, the challenge is whether we can electrify the network faster than existing DMUs hit end-of-life. If we can, then that seems far more sensible than ordering custom, GB-only DMUs that will involve paying someone a lot of money to overcome the engineering challenges. If not, then DfT should lay out when the order will be required, put out an early ITT and give trainbuilders time to address the engineering difficulties in the design stage, issue one substantial order to amortise the design-cost, and then plan the electrification so these last DMUs can get a full life-span before being replaced by electrification. They could, of course, be planned to end their lives on the non-electric rural lines before being replaced by LHCS in the 2050s or 2060s.