Some time ago an FOI (Freedom of Information) request to the UK Met Office got the following response:

Background: Class 1–2 are regarded as excellent or very good exposures, Class 3 permits uncertainties up to about 1 °C, Class 4 permits uncertainties up to about 2 °C, and Class 5 permits uncertainties up to about 5 °C because of nearby artificial heat sources or other exposure problems.

According to analyses of the released data, approximately:

22% of stations were Classes 1–3, about 49% were Class 4, about 29% were Class 5.

I guess there must be very good reasons for not ensuring that the majority of stations are Class 1-3.