Southampton City Council are following government guidelines regarding COVID-19 and the licensing department send out notifications to the trade when possible.
There is exemption for drivers not to carry the full number of passengers as shown on their identification plate. For example, if the vehicle is licensed to carry 4 people, you are quite within your rights to carry one person, the same as a multi seater vehicle licensed to carry 7/8 passengers can carry less people, it is up to the driver to decide.
As long as screens do not foul airbags, the council is allowing them to be fitted but they are not compulsory. Travel safely.