What would you realistically do with 20k?

Sure, but you get £500/£k tax free so the first £7/14k (at 7%) to £10/20k (at 5%) is tax free, and then you still get most of the taxable part Eg let's take the basic premise of this thread and say you're a basic rate taxpayer who gets handed £20k You can overpay your mortgage and save £700-1000/year in interest (assuming a 3.5-5% mortgage), although personally I'd only save about £400 (~2% older mortgage), but let's say 3.5-5% as being more representative of now and the peak rates a couple of years ago before rates started to drop Or you can put the £20k into the savings accounts I mentioned at an average rate, by my back-of-a-napkin maths, of roughly 6.3% (and then you could actually move it to the higher interest accounts over time, but again let's keep it simple and say 6.3%) So your savings accounts would get £1260 of interest. The first £1000 of that is tax free... so we're already coming out ahead of even a 5% mortgage. And then the £260 is taxed at 20%, so you keep £208 of it. Gaining £1208 is better than saving £700-1000 Even if we assume a higher rate taxpayer, £500 is tax free and £760 is taxed at 60% for £304 in your pocket, so it's still better than a 3.5-4% mortgage which is typical currently. So yes, if you're a higher rate taxpayer with a 5% mortgage from 2023 then you should overpay. But most people aren't higher rate taxpayers and most people don't have a 5% mortgage