10992
Silverstream House doesn't appear to be mapped yet, so that remains to be done (unless I've missed it).

As you say, fourth floor offices are unlikely to be useful, it might be a different matter if it was something more useful.  I left a comment on the creating changeset (https://osm.org/changeset/113021683).

Not being in London, I can't do a survey.  45 Fitzroy Street appears to be https://osm.org/way/420411287, so presumably the office(s) is/are above the ground-floor barbers.  Looking on Google Streetview (which obviously can't be used to map), the door that presumably leads to the offices appears very plain, and has no mention of any companies based there. https://www.google.co.uk/maps/@51.5238855,-0.1403204,3a,42.8y,234.09h,88.09t/data=!3m7!1e1!3m5!1sSyzEnEDVLr6Fyny3AXxMew!2e0!6shttps:%2F%2Fstreetviewpixels-pa.googleapis.com%2Fv1%2Fthumbnail%3Fpanoid%3DSyzEnEDVLr6Fyny3AXxMew%26cb_client%3Dmaps_sv.tactile.gps%26w%3D203%26h%3D100%26yaw%3D351.3953%26pitch%3D0%26thumbfov%3D100!7i16384!8i8192.  It's not clear when the building (https://silverstream-london.com/the-building/) opened, so the image may pre-date its opening.

The node (https://osm.org/node/9203994779) appears to be rather far out, assuming https://osm.org/way/420411287 is mapped correctly.