Many others have commented but I can confirm that using your Avios on any popular flight is, in practical terms, impossible. For example, just try using them to get a business class ticket or upgrade to London. Even if you accept any flight on any date as far in advance as the web site will let you, nothing is ever available. Less popular routes like holiday destinations out of season - sometimes a different story. Avios are a complete rip off.
And so is car hire via BA. Certainly in the UK. If you book the same car for the same period with the same insurances directly with Avis (without any special codes, promotions or memberships) you will get it for about 30% less than the BA price. And book it via one of the UK car hire agencies, still with Avis and same everything, and you will get it cheaper still.
They are simply riding the gullible public. It is sick!