Plans for double-decker trains for Britain were on the cards as early as the late 1930s. Stock was trialled in the late 40s/early 50s but the trains never made it to British lines - largely because tunnels and bridges constructed in the Victorian-era did not have the necessary height clearance. As both the BTC and Railway Executive were essentially running the railways on a shoe-string budget, the money was simply not there to make wide-scale improvements to accommodate this new type of train. When, in 1955, the railways finally received money from the government to invest in new infrastructure and new trains, it was largely squandered on building new steam-locomotives and large marshalling yards which saw little use.

Anyway...I'm drifting off the point now.

Here's a link to a short Pathe News Reel showing double-decker trains in Britain.

http://www.britishpathe.com/video/double-decker-train