"Non-TfL" stations may be operated by NR TOCs, but the stats for six of them will also be boosted, just as they are for H&I etc, by people changing to or from the Tube, DLR or tram.
Surbiton appears to be the busiest station in London outside Z1 which is not an interchange.
I wonder how much Clapham Junctions stats were reduced because of the Waterloo shutdown and the woes on Southern?
Waterloo's drop was probably also affected by the blockade in summer 2017.
Note that IBM has been open for two-thirds of the current reporting year. IBM's figures dropped by 87.3% last year - the largest drop in the country, and it went from 185th to 44th in the least-used rankings. (51st place to 14th in Scotland) A further 87.3% fall would bring it below 100, and would, on this year's figures, have put it in sixth place. A 95% drop would be needed to take IBM below BSC Redcar's total for 2017/18.