The cheapest standalone GPS I can find in London is the Nuvi you mention. This is €120 at present exchange rates, so isn't cheaper than Wayfinder in the UK. I agree that I have already bought the IT but the GPS is just a convenient addition. I wouldn't have bought a standalone GPS.

I use my N810 in my vehicle, rented or not. Certainly in the UK, you'd be a mug to leave a GPS in a car!

The Nuvi in the UK only has UK mapping. Wayfinder covers Europe and the world within the price. That will be expensive to do on the Nuvi.

Mapwise, Wayfinder is as good, if no better than others (2) I've tried in the UK.

I'm not an advocate of Wayfinder, I just find it does what I need with no problems and is not particularly expensive for what it offers.