The consumer rights act gives rights to purchasers from the retailer - not the manufacturer. If I’m right in my assumptions you bought the radio from a retailer and have approached the manufacturer for recompense under their ‘extended manufacturer warranty’ rather than approaching the retailer under consumer protection legislation.
I had a look on the Majority website and cannot find the terms of this warranty - just this which doesn’t really help as it doesn’t tell you what they will to do in the case of faulty items - “As part of the extended warranty, you will be eligible for our free returns policy up to 2 years after your 1 year retailer warranty period. We supply you with our address and returns information so that you can arrange your own return. Please return all items provided in the box on purchase, this includes any accessories, cables, and/or remotes.”
Manufacturers warranties are non statutory, so you are at their mercy. Your best hope of recompense under consumer rights statute is to argue your case with the retailer. But it’s your responsibility to “prove” that the item should have lasted longer - in effect you need to prove that the fault that has manifested now was present when purchased.
You’ve been offered a 50% refund from the manufacture on a three year old radio; like you I’d prefer a replacement but it may be difficult to persuade them to do more. And it would be equally difficult to prove to the retailer that the fault shouldn’t have happened after three years use. Even if you could, it would not be unreasonable for the retailer to counter claim that the expected life would be 6 years and offer you a similar 50% refund.
I’d be inclined to try again for replacement from the manufacturer, but if they won’t yield, grudgingly accept the 50%.