Delay repay, is the UK compliance with eu law.
It was previously mandated in 1371/2007, where it stayed until Brexit. It is now within 782/2021. And as it eu law, it has been retained in UK law, and will be so, until it is specifically revoked or overwritten. The UK law needed to create GBR (and overwrite the retained eu laws of railway privatisation) will no doubt update the delay repay policy, and retain it, though perhaps less generously.
I must note, that as with most eu laws of this nature, it sets a minimum standard (for delay compansation), and as long as that eu minimum was met, a member state was free to offer higher levels of compensation, and do so at lower thresholds. Which is what the UK did. Spain is also particularly generous, with its high speed AVE delay compensation set at 100% of the fare for a delay of 31 minutes or more, though it is set at 90 minutes for other services. Incredibily, RENFE used to give a full refund for delays of just 5, yes FIVE minutes of delay, to trains on its original Seville to Madrid high speed line.