It really very much depends on the school. Did you have a chance to talk to the head of games when you visited? Or can you get in touch to ask about this?
Some schools really do want evidence of high level via clubs, country squads etc, but others don't and assess on both potential and achievement. You can only find out be asking. I've a vague impression that some schools are only interested in pre-formed high achievers, whereas others are confident in their coaching standards and are looking for potential and attitude.
You might like to ask whether they do have a programme for their scholars in particular and/or their top squads generally. Or is it just capture some strong performers by a nice discount to the fees, but offer nothing beyond their ordinary PE curriculum. There's a fair amount of variation. Also see how many teacher and coaches there are in the games department, and how many fixtures they have and how many teams they can put out.
Typically, you need to show a real strength in at least one of the main sports of the school plus other sporting interests. So if it's a school that has gymnastics as a major sport, then being an excellent gymnast plus generally sporty may well get them through the door for the assessment. Some schools will consider the good all-rounder - the type who is A team for everything, has committed participation (outside of school) and is fit/strong/co-ordinated. (They may also have an eye on whether the truly outstanding at county/national level will genuinely be available for weekend fixtures and early morning or after school coaching).
Also there may also be fitness tests (has your DC ever done a bleep test?)