I have heard that the reason many British beers have relatively low alcohol content (<4%) is due to limits imposed during WW2. Is there any truth to this?

No, not to my knowledge. Historically, English beer was taxed based on the grain bill that produced it. This system of taxation dates to the late 1700s.: "The late 1700s saw a system of progressive taxation based on the strength of beer in terms of cost of ingredients, leading to three distinct gradations: "table", "small" and "strong" beer. Mixing these types was used as a way of achieving variation, and sometimes avoiding taxation, and remained popular for more than a century afterwards."

Of course, less grain would be linked to a relatively lower alcohol content. However, to answer your question, the origins of relatively lower alcohol content significantly pre-dates WWII.

In formulating this response, I also considered the question of how rationing in WWII impacted Britain's beer production. This article indicates that brewing was not curtailed during the war, and was supported (from a resource perspective) as part of the war effort.

My bona fides: American beer industry experience, an undergrad BA in history (no beer history focus though), and an amateur interest in beer (homebrewer and beer history book consumer).