I just had a quick look - it seems you can just use a seat belt in the UK to anchor a car seat - you don't need (legally) to use a top tether strap/anchor - you might need to check if they have an available anchor point in their car if you want to use this feature of your Aussie car seat. They are in the process of introducing the ISOFIX system for anchoring seats because apparently so many (70%) are not installed correctly using the seat belt.