i-Spin XL

There's different regulations in the UK for car seats so most/all American seats might not fit. Depending on the area of the UK you are planning to visit you might find public transport is a better option than renting a car and definitely a safer option than renting car seats that you can't guarantee haven't been thrown about and damaged. If you are planning on coming back then you could buy 2 inexpensive seats to use next time there is a joie seat that's both HBB and 5 point harness forward facing to 18kg (the weight limit is the only limit and must not be passed) they are around £40/$52 a seat

we have the XL version and it kept my 2yo completely safe (read facing) when the car went head first into a tree and totalled so i trust it! however, for OP, i would not want to be moving it from car to car every few days 

hold two pieces together but they fit really well and i think are magnetic. I've never had any issues clipping them in at all if that helps

The Besafe Stretch is excellent for extended rear facing. The Joie ispin is great and very handy but only goes up to 18kg

We swapped to the joie ispin when baba was about 4 weeks old. (The bucket seat we got with our travel system just wasn't doing it for me) I love it. Installed correctly, its completely stable, my boy is 1 now and 99th centile. I cant imagine having to wrangle him into a car seat that doesnt have the spin feature