Harrods offer a great selection of Glasshutte Original but you will struggle to get a discount.
Wempe on Bond Street also have a good selection but again a discount will be a problem.
To get a good discount you need to either buy from a grey dealer in the UK or take a weekend trip to Europe.
I took the Europe trip option and came back with this
http://i65.tinypic.com/mwql9k.jpg
This was last year prior to the Brexit referendum when 52% of the UK population voted to make themselves and their children poorer so the exchange rate was around 1.38 Euros to GBP plus I got a large discount. You would still be able to get the large discount but naturally the exchange rate is much worse now.