Unfortunately, age or value of contemporary/similar bikes is no indication of value. People tend to get nostalgic about the iconic bikes of an era ie the ones that were on TV ads/shows/movies, printed ads etc. Hence some "kids" bikes eg choppers, bombers in good restored condition might go for more.I could imagine the Magic going for a bit more than £20 as it is quite appealing looking and indicative of an era, but only if someone is looking for one (I don't recall ever seeing one, and I suspect I'm old enough to have been a kid when they were new). The plastic cones on the wheel nuts are weird as they feel like a later addition.
I tend to think that kids bikes overall have less value because the collector cannot ride them (unless they are a Nathan Barley type).
I'm no expert in collecting kids bikes though, and if you think they are worth more - pop them on eBay and see what happens. At least being smaller, they should be easier to post.