This app was mentioned in 1 comment, with an average of 1.00 upvote
I find watching kids use apps is one of the best ways to see what works and what doesn't. This one surprised me. https://play.google.com/store/apps/details?id=com.wilddevilstudios.sightwordsracing We have tried a few, but this is simpler and also works better.
As you can tell not GCSE level.