Good advice from CDA there. I'd certainly not profess to be an expert in personal finance, but fixed term bond investments (which are available from most high street banks like Barclays) seem to offer a good return. If you're doing this though, it would be worth checking what the underlying bond is invested in.
CDA: not sure I'd agree that the £25 billion is a sunk cost. It's an asset, of which the value can be debated but certainly an asset. The P&L amount of £433 million is the interest on the mortgage book (plus other businesses) and doesn't correlate with the repayment profile which you'd expect to see in the cash flows and balance sheet.
Using your car analogy, I'd say it's rather like buying a car for £10,000 and selling it to someone for £10,000 plus interest, payable over say 10 years. The interest is the amount you'd book each year through the P&L, and the amortisation of the loan would go through the cash flow. At the end of ten years, you have your money back plus interest. If the other person defaults then you have the car plus the amount paid to you to date.
NR situation is not dissimilar. Assuming they take on no new business (which won't be the case), that £25 billion would amortise over time during which time they would be receiving interest. If after say, 2 years, the government tries to sell it, they'll sell it for market value of mortgage book, plus the capital and interest they've received in the meantime. (I think that's right anyway)