YANBU. For those who can afford it then they could give lump sums separately (with or without identifying themselves) and not squeeze out Joe Public who doesn't have tens of thousands, but would like to drive a special car one day...
The very rich can drive a Ferrari any time they want (and fly to Nevada or Utah - one state has no speed limit, just expect drivers to "go safely and be aware of road conditions" so on a good day with high visibility you can push a car to its limit, as long as you are not endangering anyone).
If they want to publicise their business they could have a "presentation size" cheque and get their own photographer to snap them handing it over to a local BBC radio station manager (or one of the presenters if more photogenic!)
What annoys me most are where there are costly telephone charges and the networks are unwilling to donate any of the chunk they receive.
Some number I heard recently had only 80% of the cost going to CIN. BBC doesn't take anything but promotes it everywhere (including The Archers) so who gains? Network operator. BT often supplies call centre staff (heck, bet they would be twiddling their thumbs that evening anyway!) but AFAIK, is not the supplier for other services to BBC.
BBC does not pay 'extra' for the short codes (88291 85058 etc, and 81333 for local radio stations) but those are not within any 'bundled' text allowance (and if you do pay, they can be 4 times the cost of a normal text to another mobile or landline).
BBC was criticised for adopting 0870 numbers a while ago (esp people behind SayNoTo0870) and dismissed it as the deal they get is for the network to supply them at short notice, be flexible and give them nice numbers, so the BBC think it works well for them.
Network operator rakes in money as a result of this 'back scratching' deal and who pays ? Us.