The cache is not at the posted coordinates, which willl bring you to the corner of Regent Street and William Street . There you will find a post box and a couple of benches. To locate the final coordinates simply answer this puzzle and put your one-word answer into Ceritude .
Puzzle.
What relation was the person, whose Royal Cypher appears on the post box, to the person for whom the street has been named?
If you have completed the nearby Adventure Labs Newtowne in the Ardes. you will have learnt about Regent Street.
See https://adventurelab.page.link/zKwS
You can validate your puzzle solution with certitude.