Got me thinking – so checked up.
Apparently, contrary to what I thought, it's specifically NOT against the law in the UK to have an 'unrestrained' animal (by way of leash, cage or whatever) in a vehicle.
However – Driving with pets is covered under Rule 57 of the Highway Code that states: “When in a vehicle make sure dogs or other animals are suitably restrained so they cannot distract you while you are driving or injure you, or themselves if you stop quickly.” However, don't be fooled by thinking this is just advisory as its 'only' in the highway code. Legal guidelines indicate that if an unrestrained animal is involved / causes an accident the driver may be penalised for driving without due care & attention, fined up to £5000 (+ boat load of penalty points). Also both vehicle AND pet insurace may be invalidated, so you could end up severley broken financially as well as bodily.
It gets more complicated – be aware there are apparent risks relating to passenger seat airbags if letting (even if restrained) animals sit on the front seat. We don't so doesn't apply to me, but thought it wise to mention it in case anyone does.
Hope that clarifies things a bit. Regards Mike