This one certainly poses some ethical dilemmas. LNER's website says
"How do I apply for compensation for my cancelled or delayed journey?
Cancelled train
For cancelled trains, we offer full refunds if you did not travel on another train. You need to apply for your refund within 28 days of the cancelled train date"
If you did travel on another train, then Delay Repay applies to actual delay experienced ie you would only get 50% back in this case.
So, you could write in and say that you should only have got 50% back, through Delay Repay, and offer to pay that much back.. Or you could regard it as LNER offering you more than minimum compensation under its rules (ie, in effect, an early Christmas Present). Perhaps reclaiming only 50% and then using the rest to make a donation to a charity called CALM (the option that LNER's website offers) might be a way to go particularly given the time of year? We could go on to debate whether you could get Gift Aid on it, even though you won't have paid any tax on receiving the payment (as it was compensation). But there are probably better things to do!