In the US and much of Europe one can flightplan on the presumption of a class D transit request being granted (from memory, in the US you only have to establish 2 way comms). In the UK, one has to pesume it will be denied or delayd "due to controller workload" and plan alternatives. As a result most VFR pilots will avoid Class D as a matter of course.
The situation is probably better now than it was a few years ago and there are notable exceptions , but this requires local knowledge.
Remember that UK airspace is controlled by a company 42% owned by the airlines. It is not a public asset for all to share.