My guess would be that this is matched betting of some sort which is indeed tax free and fully legit.
Yes, if it is matched betting, you
could pay the OP £30 to learn about it, or you could just read
Wikipedia for free and do a bit of research about which bookmakers still offer free bets. If you're reasonably clever and good with numeracy, you should be able to figure it out on your own.
For what it's worth, I've got a mate who gave up their full time job to do exactly this, so it must be profitable. He tried explaining it to me in detail, but there was alcohol and loud music involved at the time, so it didn't really register. If I wasn't indolent, I'd probably be doing it myself.