In both cases though I would have though you'd have to get them blown up the same amount each time or the seat belt would not fit correctly.
I'm hopeless at blowing things up, not much use as a small travelling seat if I need to bring the air bed pump too.
Trying to imagine the taxi drivers face "hang on a minute I just need to blow up my kids car seat"
I would not want to put my 3yo in a booster but most people here (UK) do. In fact when we were looking for seats for DD (now 6 so about 4 years ago) there were several high back boosters marketed as suitable from 9 months
For taking to restaurants to let kids sit up higher to eat.
Now, from a car perspective, what that "seat" would do is give the appearance of a seat to avoid fines and tickets. Y'know for all those people who care more about not getting in trouble for endangering their kids.
Although, they do claim it's been tested and approved for 15-36kg kids. Is there a list of approved U.K. seats somewhere to cross check?
Totally legal in the UK and europe to have a 3yo 15kg child in a booster seat. Did I do it - no, bit a lot of parents do. Higher weight harnessed seats are not available in UK at an affordable price, and US seats are illegal. The only legal higher weight seats I know of are the Swedish RF seats which are not cheap and not widely available, but availability is improving: http://www.rearfacing.co.uk/buyersguide.php. I have not seen them anywhere in Spain.So most parents are going to go from grp1 seat ((ff 9-18kg) to a grp 2/3 seat (15-36kg).