Having regular training is a good idea, but testing every 5 years to drive at a max speed of 20/56mph will be disastrous for the economy. Fortunately the only good thing the greens will do is split the labour vote.
Magistrates will be clogged up with speeding offences.
The policy is gated/members only, but there are ways around that:
TR113 The Green Party would introduce a 20m.p.h. limit throughout built up areas, including villages. This overall approach would reduce the need for specific traffic calming measures everywhere, but it is recognised that there would still be a need for these in many locations, especially the entrances to these areas.
TR114 In rural areas, apart from trunk roads, the maximum speed limit would be 40m.p.h. Local communities would be encouraged to set lower limits on country lanes where pedestrians, cyclists and horse riders would be particularly vulnerable. They would also be encouraged to designate green lanes where these modes would have priority, and where feasible, to close lanes that act as through routes to allow only for these modes plus local access.
TR115 On major roads outside of built up areas, the maximum speed limit allowed would be 55m.p.h., to maximise the efficiency of fuel use as well as improving safety.
https://policyarchive.greenparty.org.uk/policy/transport/