Dude1: Personally, I am unaware of the policy in the UK.
In the UK, it depends on the dealer. They can choose whether they'll accept something or not and how much they'll offer. However, if you're wanting to purchase a substantial amount of kit, the trade-in deal might be better than you'd get selling personally. Better still, in the UK they'll come around and pick up the trade-in. You don't need to do a thing!
However, one of the biggest plus-points of B&O kit in the UK is that they usually give you a fair trade-in price for old kit. Hell, I know someone with a radio from the 70's and B&O said they would accept it as a trade-in against new kit.
The kit is then available from the store as 'pre-loved' hardware. Most stores will give you a list of their current pre-loved stock. Obviously this stock still has a mark-up and is ideal for those who want B&O kit, but can't afford brand new hardware.
If this service was removed from the UK - as it is suggested it had been removed in Australia - it would be a key reason when considering new kit in the future. I'm not saying I couldn't get a fair price for my BV7-40 in a few years from now, but I wouldn't have a clue how to dismantle and move this kit....so it's far easier to trade-in.
BTW, you say 'try it with any other manufacturer'. Fair comment, but bear in mind that this is a service that we thought was standard in the UK and was one of the reasons for us making a purchasing decision.