Tim,
I would advise looking at the following dealers websites and then contact them directly. Give them two options, you will take what they have or are prepared to wait for a factory order depending upon how long a new build will take. This is typically 6/7 months depending when a dealers build slot is. Discount may be offered but I suspect you are more likely to get a motor mover as an incentive to buy.
Lowdhams seem to be selling a good number of vans and have bases in the Midlands
Becks Motorhomes are based in Norfolk and new this season to Eriba caravans
Automotive Leisure in Dorset are the largest dealership (possibly the most expensive).
Eriba caravans, Jandi of Scotland are the longest trading dealer with a wealth of knowledge, but I'm unsure if they hold much stock
Secondhand/ex display it would be worth contacting Dom of http://www.eriba-shop.co.uk/ in Norfolk. He sources vans from abroad where the stock is far greater and a dealer is more likely to have an end of season van standing. For a dealer his prices are low and his reputation very good.
Best of luck with the search - Colin