In London you better off going to the Dior Outlet which is located near Oxford in a nearby town called Bicester, or for full price Dior just visit the Boutique on Sloane street, Harrods and Harvey Nics are close by who also stock Dior.
Selfridges is down oxford street carry Dior. Also the New Shopping Centre Westfield have a Dior boutique.
yes you must visit the outlet, they have 30 - 40% discount and sometimes you can get an extra 10% VIP card from the Shopping Centre reception. This entitles you to 10% off all the shops within the village.
Even with this discount its not comparable to the outlets in the US.
I believe here in the US it is standard to mark everything 50% off from the retail price, plus during holidays or if they've only got a couple of bags from one line, they give an extra 20%. Makes you wish you could buy everything in the store.....