If the 'Frenglish' title is a clue.... could it be South Kensington, the French Institute maybe? (complètement wild guess... :-)

Merpeople wasn't meant to be frenglish or even franglais! As far as I know there isn't a French connection but interesting speculation.
Not a civic building and not particularly close to the Thames

I'm not sure what it symbolizes. (Trying to find out!). Can't think of a Nordic connection though. The Waves might have been conceived in this area...

Bloomsbury, yes, where they lived in squares, moved in circles and loved in triangles. Take your pick of the three.

I agree with Jim... Bizarrely, I always thought of the Bloomsbury crowds as being into art and literature and not so much maths or geometry, might have to re-assess!
No idea about where this is though it does look familiar...

Think you've got your triangulation wrong, Jim! Believe Geographia (and also above I did suggest that you selected either square, circle or triangle...). Please now type correct guess into box below to be awarded the point!
I still haven't worked out what the relief is meant to symbolise though. Will post update if I ever find out...

Jim, I beg your pardon. The building is Tavistock Court which is in Tavistock Sq - or at least, it faces the square (on north side) but I think the road might be called Endsleigh Place. I was convinced you'd tracked it down before but forgot that the building itself was called Tavistock Court.
I shall award you double points to make up for the confusion I have caused (and give myself a slap on the wrist...)