en.m.wikipedia.org
Easily one of the more consequential what-if's for the British economy in the 20th century, especially if it is completed when it was first proposed in the 40s. Widening the canal system would allow barges to move goods from the industrial centres in the Midlands and North to large swathes of Continental Europe. This replaces multiple choke points (loading onto rail, unloading, loading onto ships) with a smooth distribution system.
It also has the added benefit of weakening the monopoly power of the unions (railway, dockers), and gives companies options for transporting goods. A strike in one sector would not have the ability to strangle the British economy in the way that it did (the 1970 dockers strike being a good example).
As it happens, contrary to popular opinion, I think a post-1900 departure makes the most sense. If you get a pre-1840s expansion, those canals are going to be irrelevant a century later. After rail comes onto the scene, it is the most cost effective method of transport within the British Isles throughout the Victorian era, and the British economy is geared towards the Empire much more than Europe at that point. I don't see the Victorians building canals on a huge scale.