This was the basic Hercules utility model in the '50's and is still relatively common, so not a grail bike, but a good bike for someone who wants something vintage and a little different. Condition counts for a lot on these and this one looks better than most. I'm curious if there are any markings on the rear hub, most of these frames came with Hercules hubs, but according to Grace's Guide to British Industrial History:
"1956 Tube Investments made 1,250 employees of the British Cycle Corporation redundant, following deadlock with the unions, who refused to reform working practices. Many of the workers were from
Hercules factories"
The Hercules bikes I've seen after 1956 have different frame lugs than yours and the Hercules-built hubs were replaced with rebranded Sturmey-Archers, so if yours is from 1957 it must be one of the last.
www.gracesguide.co.uk