I'm very good friends with Britax and understand the answer. Their car seats are excellent but you also have to understand that people at customer service have little or nor experience with car seats. Especially rear facing seats.
Seat belt, including stalks, are incredibly strong. On top of that there is an incredibly large safety margin. Simple answer is that regardless how much you twist the stalks they are still very strong strong. To show how strong the belts are we can look at a practical example. Lets say a normal European male driver weigh 85 kg (187 lbs). Around 275 lbs for a normal American.....:-) In a frontal collision, the most common, at only 50 km/h (31 mph) the European driver will weigh around 3 400 kg (7 480 lbs). The seat belt will halt the driver and easily handle this huge weight. Keep in mind that this is a collision at only 50 km/h. At higher speeds the forces are of course far far higher. Even at collisions at Autobahn speeds the belts are strong enough.
A car seat might weigh 10 kg, give or take a few kilos, with a child weighing maximum 25 kg rear facing. A child in a booster might weigh max. 36 kg plus weight of the seat which is almost always far less than 10 kg. As one can see the forces are not even comparable to the forces the belt must handle with an adult.
To find some official comment we actually have to look at US. Interesting since car seat safety there is like most other countries incredibly far behind. On top of that we also have the ridiculous legal system where people sue for anything and everything. From the Federal Register:
"The agency acknowledges that the CPST curriculum permits caregivers to twist buckle stalks in order to achieve a tight installation or to prevent buckles from resting against the entrance to the belt path, as long as the CRS and vehicle manufacturers both allow the practice. The agency has received data from Indiana Mills & Manufacturing, Inc. (IMMI) that indicates no considerable reduction in the strength of the seat belt webbing is observed if a flexible seat belt buckle is twisted three times; therefore, twisting the seat belt buckle three or less times is considered acceptable practice and is often necessary to achieve a tight fit. [39] The agency believes, however, that this practice is not well-known to the average parent or caregiver. In addition, many buckle stalks in the vehicle fleet cannot be twisted due to rigid plastic coverings. Some child restraints have higher belt paths than others, which can eliminate the need for twisting the seat belt."
In summary, twist your buckle stalk up to three full laps. Britax Max-Way, Britax Hi-Way and Axkid Minikid all work quite well as long as the Britax wedge also is used. The wedge will lift the seats higher off the vehicle seats.
Floor compartments is not an issue. Fill it with styrofoam or place a larger solid object on top of the compartment. VW is using styrofoam for their filler. Old phone books would for example work nicely. Or palce support leg on botton of compartment.
Without getting into too much details we clearly see that the floor in a vehicle isn't that solid even without compartments. To minimise weight and provide space for cables, wiring, etc the floor is like a Swiss cheese.
I don't want to offend but I would respectfully say that having a tight car seat budget at 200 GBP is extremely irrational and without logic. In the big scope of raising a child the amount of a car seat is nothing. A good rear facing car seat is the best life insurance available. We know this since traffic is the number one killer of young children in most countries.
Cut down on cigarettes, food, travel, vacations, clothing, cable TV, pub visits or pretty much anything else. No to mention that a car, even a used one, with insurance today cost lots of money. Placing a limit on a car seat is the absolute last thing one should do.