Jon,
The frame you have in question may be made from Accles & Pollock tubing, a similar alloy to Reynolds 531. The steering column with have an A & P stamp on it. But considering the rear dropouts, this frame may either be a low-end bike OR one made earlier than the 60-61 date in the listing. Campagnolo began distribution in GB in about 1957. Until about 1960, their dropouts were optional on high-end bikes. I would also ask the seller for a CU photo of the rear dropouts. A popular brand of the day was something beginning with an "A." Too late to look it up.
Michael Allison
New York, NY