Every Stage

We bought a Joie Every Stage specifically as our "travel" car seat, though our child is younger (was 19mo) and I wanted ERF. There are definitely lighter weight options for older children.  If you have the storage space for it I'd buy dedicated seats for travel, assuming this won't be your only trip they'd pay for themselves over hiring. This way your partner will feel comfortable knowing your main seats haven't been in cargo. But you're not at the mercy of unknown hire seat standards.

We’ve got a Britax Romer Max Safe Pro which is what is used 90% of the time because its in my car, its a really good seat, scores high on safety tests, easy to fit and will last at least 4 years which I think I saw you say you want. They do sales often too so you can often get it for around £180. We also have a Joie Every Stage for my husbands car which works fine, but I do prefer the Britax