If it's still there, it is likely to be a minicab office (where you can walk in, book and wait for, a Private Hire Vehicle). There's very limited use of office=taxi and some will be mis-tagged as amenity=taxi (both unsatisfactory as PHVs are not the same as taxis in the UK).

A new tag is needed for minicab offices, as they're a common feature in the UK.

I'll check the POI next time I go for a swim in West Reservoir.