It means that a lot of national-running trains have to be diesel.

Or be multi-voltage, as most modern electric trains (at least
potentially) are - Electrostar, Desiros, 319s, 365s, EMUs Silverlink
and WAGN use in London, Eurostars.

I can't think of any services which are diesel because of the two
electrification systems. They are diesel simply because of a lack of
electrification.

I can think of some which used to run: the North-West to Brighton
services via the Trent Valley and the WLL.