The blurb above the table notes that the ranking is "the number of destinations their holders can access without a prior visa," and that it is based on data from IATA. Basically, the same data as used by airlines to assess documentation requirements before allowing passengers to board international flights.
The UK is above Belgium because, according to their data, a British passport allows visa free access to 187 destinations, while a Belgian passport only allows access to 186 destinations.
The UK is above Belgium because, according to their data, a British passport allows visa free access to 187 destinations, while a Belgian passport only allows access to 186 destinations.
There's also a full methodology page at https://www.henleyglobal.com/passport-index/about