The last really long distance non stop express in the UK was the Elizabethan from Kings Cross to Edinburgh, with crews changing over half way via the corridor tender. This ended in 1962 when the Deltics took over. With modern multiple unit trains, I see no practical reason why a similar non stop run couldn't happen with a crew change on the move half way but no doubt all sorts of modern regulations would prevent it.
Population density in the UK works against such non stop runs and the development of places like Peterborough, Reading, Swindon, Milton Keynes etc. means that they are usually too important to miss.
This is the opposite of what has happened in aviation, when long distance routes to Australia and the far east used to be multi stop affairs but now we have a non stop flight to Australia and the industry would like to eliminate as many en-route stops as possible. On these very long haul services, of course, crews change over "on the move"!