I know it's not necessarily the most popular label out there for many, but if you want cheap, Merc is cheap at Brandalley right now.
How cheap? Well, you can get a Harrington for £15, although I've no idea what the quality is like on it. You'll have to take a chance if cash is tight.
Check those out, as well as shirts, t-shirts etc now – stock tends to go quickly, so don't hang around if you're interested.
Merc at Brandalley