Here in the UK, we aren't privy to crazy sales like those lucky Americans but there are a few ways to get just a little bit of money off designer handbags in the UK.
First thing's first: getting a discount on Hermes, Chanel and Louis Vuitton is near impossible in the UK and that is not what I am talking about here. As for most other brands though...
European Sites and Online Stores
I have found that Net a Porter stick pretty rigidly to RRP but I do periodically try sites like Farfetch, mytheresa or luisaviaroma. These are based in Europe and have flash sales and discounts when you sign up to their mailing lists. They also sometimes do introductory offers for first time customers.
UK Department Stores
Big department stores like Harrods and Harvey Nichols do discount weekends where designer bags might be included. In the past, I have managed to score money off sought-after styles from brands such as Givenchy and Bottega Veneta.
Join the Harrods cardholder scheme. You will then be notified of 10% off weekends and sale previews.
Sign up to mailing lists for all department stores to get notified of discounts and seasonal sales.
The main dates to bear in mind are the end of summer (usually towards the end of June), Boxing Day and Black Friday. Get to the store or online the moment they open to make sure you get access to the best stock available.
If all else fails, or before you start looking at all these options, Google the bag you want "in the UK" and see whether Google Shopping throws up any interesting results. Also, make sure you ALWAYS check the RRP before shopping around for a deal. Recently, I noticed that a store doing Black Friday had "discounted" a number of designer bags but when I checked the price on other websites, I found that the official RRP on a number of websites was lower than the "discounted" price listed by the store! So do your research and don't get swept up in sale mayhem.