Been to London for a weekend as my birthday present. So we explored the city while caching, went caching while exploring the city. Had a wonderful weekend.
The place around the sundial was surprisingly free of muggles when we collected the information for calculating the final coords. Unfortunately we somehow made a mistake in collecting or calculating so we did not find anything. On our way back to the sundial for checking our results we stumbled upon something that looked fairly like the object mentioned in the hint so we gave it a try and "TADAAAAH" there it was.
thank you for taking us here and greetinx from Hamburg
Land: Vereinigtes Königreich
Koordinaten : N 51° 30.592 W 000° 4.589