Its an urban myth put about by the shops.
The ANO, CAP393, specifies the carrying of:
"Maps, charts, codes and other documents and navigational equipment necessary, in addition to any other equipment required under this Order, for the intended flight of the aircraft including any diversion which may reasonably be expected.
for
Aeroplanes flying for purposes other than public transport."
The key words being "necessary... ...for the intended flight"
In other words its up to you as the aircraft commander to ensure that you have appropriate means of navigation. The proverbial Little Chef map might be OK in some circumstances. I regularly fly in the local area with no chart at all - not needed.
Its also worth pointing out that there is no such thing as a "current chart". They are out of date by the time they are printed - unless you religiously mark them up with all the published amendments.