Lots of websites assume you are located in the country where your IP address is, and thus apply the rules for that country. So perhaps if someone wanted to use heuristics instead of uploading an ID, they could pretend to be in the UK by using a VPN.
Legislatively, the UK is among the stricter regimes wrt online age verification. It's a place you want to be VPNing out of. Discord is apparently rolling this out worldwide out of their own free will, not due to legislative pressures.