I am led to believe the dealers get paid by the manufacturer for registering cars - over £1,000 for a Mitsubishi 15 years ago.
Large companies, like Nat West Bank, buy in large numbers for their managers. The discount is very large. After just 6 months they trade-in for a new one. The value of the trade-in is sometimes more than they originally paid. They can claim back the tax. Poor old Joe Public effectively subsidies these deals. It is a way of registering more cars and keeping the sales figures up in comparison with the opposition. Whether this is still the case others on here might have more up-to-date information.