Unless I am completely confused, this cannot be true. Concrete has been used since the ancient Romans while iron and steel reinforcements were only really developed in the mid 19th century.
My understanding is that the use of Roman concrete was very highly centralised in and around Rome, and tailed off dramatically towards the outer reaches of the Empire. Indeed I've seen this cited as the key factor in the technology going extinct -- basically, it was very locally endemic.
In any case, I'm unaware of any Roman concrete structures having been found in Britain, so this statement may indeed be true.
Obviously, the only version of this claim that could be true is that Sway Tower was the first building in England built exclusively of unreinforced concrete. (There were uncountable buildings built by using unreinforced concrete and stone, e.g., the third Eddystone Lighthouse completed in 1759.) But I’d still find it very surprising that people built structures in England from reinforced concrete for decades without anyone trying to do it without the reinforcement. (“The first home built using reinforced concrete was a servant’s cottage constructed in England by William B. Wilkinson in 1854.”)
https://www.cambridge.org/core/journals/journal-of-roman-stu...