Thanks
I have used an Airnimal Joey, with 24" wheels. The frame will fit in the boot and the wheels then need to go either on the passenger seat or racked on the boot. The bike rides much better than the Brompton (I’ve ridden both quite a lot) and takes standard gears etc so is a bit more flexible.
Airnimal have various bag options, which could be fitted onto a rear rack. It might be a struggle to get two bikes on though!
The other option is a Mezzo D9 or D10, which folds quite small and has a full range of gears. The Mezzo is pretty good and an alloy frame. All of these can be bought from eBay for below £1k.
The Brompton is a great bike, but the limited gear range can be restrictive unless you push about 300w of pedal power!