I was just discussing with a few friends who were amazed at this, most shops can always give up to 10-20% discounts if they need to.
Especially if it's high margin, e.g. service contracts like phones or internet have a negligible direct cost to provide you with the service, they just have fixed overhead, so if the price of a service is £15 a month, it may only cost them £0.50 to provide, so even if you pay £10 they are in massive profit.
Things that are low margin the retailer has much less wiggle room.
The other caveat that if you are only spending a small amount overall, it's not really worth the salesperson's time to bother.
If you want to be a bit ballsier, if you are in a shopping centre you can always ask if you can have the centre discount - normally every employee in a shopping centre gets a small discount in every other shop in the same shopping centre - and the individual shop doesn't have a quick and easy way of checking your credentials.
So any time you are looking at a big purchase, simply ask the sales assistant politely if they can do anything on the price, half the time with a bit of polite asking they will normally take a bit off for you as a sweetener. Even if you're buying online, you can normally get a little bit off by asking, and if nothing else it's easy to get free express delivery (got this with Tesco about two weeks ago).