I'd have no issue with the whole country being covered in class D airspace if the controllers did not restrict traffic from entering. Unfortunately that is not what actually happens. For good (although perhaps not terribly robust) reasons they prioritise CAT into their fields. They reduce the workload by keeping GA out.
Last weekend I tried to transit Teeside twice. Not below 2500' was the answer - just about possible in the conditions - but hardly a transit. We got the same treatment from several MATZ, cleared to transit but remain clear of the empty ATZ, again hardly a transit.
With Humberside making an application for Class D as well the situation for north south transits in this area could become very difficult if access is not readily available. From past experience most GA pilots know that whatever the good intentions it is always easier to refuse access than grant it. Hence the resistance to more Class D.