This chap is astride a tidy trials model AJS, it looks like a 500cc so would be the 18CS model (18 was the model code for AJS's 500cc model, the 'C' stands for competition and the 'S' for springer - ie rear suspension). The bike is fitted with the infamous AMC 'Jampot' suspension units so dates between 1951 and '57.
The Ajay looks shiny and well looked after. The rear mudguard is modified with a 'skirt' as was popular with the more serious riders - keeps a lot more mud away from your hind quarters.
I wonder where the location is. Kind of looks like a sea wall with folks walking along it and picnickers alongside.