I rarely find discounts on good quality disposable razors. But today, my local Sainsburys (Hedge End, Southampton) have loads of Gillette Blue II Disposables (all varieties) on Buy One Get One Free.
This means you can get, for example, 20 Blue II Plus Slalom for just £2.99, which is a really great price.
I don't know whether this is nationwide, but the shelf edge stickers indicated that it could be. The cheapest pack of razors is free.
I've just checked and this offer is also available online.