Firstly, denied boarding and downgrades are incredibly rare - involuntary denied boarding affects something like 1/10,000 of segments in the US - I've not seen BA's statistics but I doubt they are far off.

Secondly, there is a regulatory regime in place that requires a specific level of compensation. As such, while it certainly used to be the case that airlines would give you a 'free' change from inflexible business class to flexible (premium) economy, knowing that you would need to use the ticket on the flight you'd booked originally, that's no longer the case.

For any flight operated to or from the EU by an EU airline, a difference of 3/4 of the ticket price for the affected flight must be offered. This excludes any taxes, and there is some ambiguity around any fees that are separate from the ticket price.

So if, for argument's sake, you bought CW in the sale for £1k each way, with £200 of carrier charges and £150 of air passenger duty, if downgraded you'd be entitled to at least (3/4 x £650 = £487.50). There's a reasonable argument that the carrier charges should also be taken into consideration for an additional £150. APD is the same in each cabin but if downgraded into economy (say, £75 of APD) then you'd be entitled to that, too (but nothing extra for a double downgrade).

If a walk-up WTP fare would have been £900 each way (£1,800 return) then a fare difference offer would not be appropriate; clearly if, somehow, a walk-up fare for the cabin you travelled in was £1,025 or less (or £725 if carrier charges are taken into account) then a fare difference offer might exceed the statutory compensation but that's unlikely.