Car seat for cab trips, for three year old

[Mummagumma]

I know next to nothing about car seats, and DS hasn't had to use one since he was a baby, as we've bussed and trained everywhere (lived in London). Recently moved out of town, and until one of us has learned to drive, we're going to be quite taxi-dependent. I need something that is easy and quick to fit into and remove from cars, so we can go to places that aren't served by rural buses. Not sure where to start with this. DS is a super tall three year old.

www.kiddicare.com/webapp/wcs/stores/servlet/productdisplayA_36_10751_-1_14573_101908_10001_14568

Would something like this work? Would be easy to transport about?
Depending on minimum weights etc

Do any local taxi firms supply car seats? might be worth a ring round

These are brilliant.
www.amazon.co.uk/gp/aw/d/B003HKQJFK

Trunki boostapack if he's over 15kg.

I'm sure taxi can be exempt
From the car seat law ?

Taxis are exempt from the law, but it's about what DH and I feel happy with, particularly as it'll be a regulari-sh thing until I learn to drive. DS rode in black cabs in London whilst in his pushchair all the time, but taxis that are regular cars, on country roads - just not something I'm comfortable with except in the most essential of circumstances.

You'd think some taxi firms would have car seats they could pop in, but so few do! We came to this area on holiday when DS was 18 months, and it was such a palaver trying to track down a car seat for the trip from the station to the seaside, although one firm came up with something (it was really inadequate though. The four or five times we've had to take DS in a mini cab, the car seats have looked like they've been made of cardboard, held together with ribbons).

That Trunki thing looks promising! I managed to get DS on the scales tonight, and he's 16.7kg, which is the size of the average 4 year old. How have I not come across that before!? Looks great.