They do that all the time anyway, whether or not there's new legislation. They don't want cars languishing in fields as they deteriorate over time even when not used. Dealers need a steady supply of cars to sell otherwise they don't make enough profit and may move to a different franchise/manufacturer. The car factories need to keep car movements flowing - they can't sit idle or work below optimum output efficiency. Cars sat in compounds and fields are costing interest and tieing up working capital. And, at the end of the day, the whole chain makes money out of servicing/repairs/replacement parts - a bit like the ink jet cartridge industry where printers are sold below cost to ensure buyers end up paying stupidly high prices for cartridges. So, retailers put pressure on the manufacturer to provide them with popular/in demand cars to sell and with scope to discount. Then manufacturers put pressure on retailers to sell the less popular cars. What no one wants is a load of cars stood in a field. Manufacturers "allocate" cars to retailers/dealers whether they ask for them or not - it's part of the franchise agreement for garages to have to accept what they're given and then ultimately pay for them after a number of months (usually 6 or so), hence a dealer can be stuck with cars they have to pay for but never wanted in the first place and can't sell - so they register them as demos, use them for courtesy cars or for the salesmen's "company car" and ultimately work hard to flog them as soon as they can, with whatever discount they have to give!
There is nearly always a good deal to be had if you are willing to haggle hard and walk away. Usually, the best deals are available if you can be persuaded to take a car sat in the dealer's compound or which they've registered as a demo. I.e. you go in and ask for a red 1.2 SL and they offer a good discount on a blue 1.1 SX (simply because it's sat in their compound and had to be paid for next month whether they sell it or not!). Occasionally, the manufacturer will have too much stock of a particular model and will offer big bonuses to dealers to sell them - that's not just cars in fields, but also cars on the production line if they're just going to end up in the field once made, so you can still get cars with factory fitted extras at discounts. Then at certain times of year, the dealer will be just short of their quarterly "retro bonus" targets, which are usually something like £500 per car for the first 100 registered and then £1000 per car if you register over 100 - that's on all cars not just the 101st, so if a dealer is on 90, they'll offer huge discounts to sell another 11 - retro bonus on 90 would be £45,000, but retro bonuses on 101 would be £101,000, i.e. £56,000 more, or £5,090 per car between 91 and 101 - so the dealer can offer a further discount of up to £5,000 per car just to sell them and hit the target and still make more profit, (plus the earnings on the PDI, and future servicing/parts).
(I used to be the accountant for a main Peugeot dealership! - some of the wheeling and dealing the sales director had to manage was quite spectacular to find ways of hitting the targets and keeping our own stocks low).
That knowledge/experience has served me well. I have never paid more than 75% of list price for any new car I've bought, and that's over a number of brands, inc Citroen, Renault, Peugeot, Rover, Nissan. The last one I bought was particularly spectacular, 5 years ago. My wife's needed replacing, so we went to the Renault garage to look at a ex-demo, 9 months old, 250 miles, which was priced at £15k against a list price of £20k. All fine except she didn't like the light coloured interior/upholstery. Salesmen offered to knock off another £1k to take it. Wife still not interested so we walked away. Phone call next day from the sales manager saying he could do it brand new, darker upholstery for the same £15k. I said we'd have it if he'd do it for £14k and throw in £1k worth of factor fitted extras (parking sensors, etc). He said no, so that was that. Then a week later, he phoned back and said yes, he'd do it, so, then I asked if he'd throw in a "proper" spare tyre (instead of the temporary one that comes as standard) and floor mats. He agreed as long as it could be registered by the end of the next month (so just enough time to get it through the factory and delivered), so I got a car with list price of around £21.5 (inc extras) for £14k. All because I knew it was "that time of the month" in respect of their retro quarterly bonuses and they were desperate to register as many as they could.
Whenever there are other factors, such as a model coming to the end of its production life, or change in legislation, they're even more desperate to get rid of stocks.