[Emyr]

I could be wrong here - but I've never come across any law in the UK that prevents a private non-goods vehicle from being driven on the roads if it's over it's manufacturers specified fully-laden kerb-weight?

I've an old colleague who drives a retired ambulance around - modified by its conversion from van to ambulance with around 3 tonnes of extra auxiliary batteries, hydraulics, oxygen tanks and ballast weight. There's no law stopping him from driving that!

You and your colleague are both wrong.

Whilst discussion of overloaded vehicles is easier to find in the context of goods vehicles, the law also applies to passenger vehicles.

The Road Vehicles (Construction and Use) Regulations 1986
http://www.legislation.gov.uk/uksi/1986/1078/regulation/100/made

Road Traffic Act 1988 Section 40A
http://www.legislation.gov.uk/ukpga/1988/52/section/40A

These are both referenced in this DVSA enforcement document:
https://assets.publishing.service.g.../795005/dvsa-enforcement-sanctions-policy.pdf

Look for PLS codes 1-55 and 1-56.