Employed services of 'architect' but found the person is not an architect, can I cancel contract? (UK)
Yes, you are entitled to rescind the contract.
In terms of the Restatement (Second) of Contracts at §153 (US law), your basic assumption was the belief that that person is an architect. That belief is "not accord with the facts", Id at §151, his silence and inducement in that respect are tantamount to misrepresentation, and at no point did you purport to disregard the possible or actual inaccuracy of that belief, Id at §154. Therefore, the contract is voidable by you.
Also under UK law "[t]he normal remedy for misrepresentation is rescission [...] Rescission is prima facie available if "practical justice" can be done". See Salt v Stratstone Specialist LTD , pars. 24, 30.
He not only knowingly failed to make the pertinent disclaimer or clarification despite your multiple references to him as "architect", but his written --and thus readily provable-- statement "Thank you for inviting me to quote for the architectural work" evidences his willful reinforcement of your belief that he was an architect. That is because an architect is presumed to be the right individual for devising a quote for architectural work.
This person's statement and the circumstances you describe fall within scope of the Consumer Rights Act 2015, s.50(1) (not that subsection (2) is equivalent to the Restatement at §154 and therefore inapplicable to these circumstances). His inability to satisfy your implied requirement that the job be performed by an architect, coupled with other significant flaws you identified, indicates that he cannot comply with the terms of the contract. Accordingly, the situation warrants rescission.