Iggi the problem with buying car seats when you arrive in the UK is getting the child and the hire car from the airport to the car seat shop!
Car hire companies royally rip off anyone hiring a stage 2 car seat, charging more than a seat would cost new for a couple of days hire of seats that look as if they are not fit to be used - hard plastic barely covered with a scrap of not clean looking ragged cover IME.
Are the Trunki seats not HBBs?? I've thought of buying one for my youngest to avoid lugging a regular hbb (if we visit the UK this year I'll be on my own with 3 kids, and the advantage of a truki is that the child carries it themselves as a backpack) and thought they were hbb - obviously if they are just booster cushions that is a bit pointless, as carrying a booster cushion isn't that much of a problem compared to carrying a great big hbb!
Has anyone else looked at the label on their hbb? I did notice that one of ours actually said that after 25kg it should be used as a cushion - that was a stage 1/2/3 seat which had a label stating when to change between set ups. The other seat we have is good til 36kg but by 140cm tall DS (who does have broad shoulders) was not comfortable in it and was sitting in a cramped position - which was when he moved to a cushion.
A booster cushion does not offer the protection of a hbb but people saying they are useless are also being a bit dangerous as they do improve the fit of the seatbelt for bigger children in the 140-150cm height band. Few parents in reality will go out and import one of the very big XL seats from America when a child is already 140cm tall and not fitting into the hbb they have been using for 3 or 4 years, so kids over 35/36kg are realistically going to be either on a booster or nothing - and discounting the booster cushion is quite foolish unless in the odd car where the regular seat belts adjust down very low.
I wonder if everyone who has multiple kids in hbbs at the same time has their hbbs fitted properly too - lots are not meant to go in the middle seat, so 3 hbb across the back only works in a very, very limited number of cars with an equally limited range of hbbs. I don't claim to know whether a hbb in a poition not approved by the manufacturer is safer or less safe than the tallest middle seat child being on a booster.