Sign up to Which? magazine subscription for £1 and instantly you'll receive a £20 discount voucher via email (off orders over £150). This can then be cancelled before your sub trial ends. Use the voucher for the Dyson. I checked everywhere and this is the cheapest I could find.
Dyson Outlet on eBay are selling the same V6 model for £179.99. But they are refurbished models.
So you pay:
£1 Which? trial (cancel afterwards)
£174 Dyson V6
You can also put the deal through Quidco for 2% cashback which makes the total £170.52 if you don't mind waiting a few months for a few quid :)
Link to info on Which? trial: http://www.appliancesdirect.co.uk/content/which-promotion
NOTE. You have to sign up for the which trial via the link on the appliancesdirect website. I was wondering why I signed up directly a couple of days ago and didn't get a voucher. that's why.