Asda in-store Pharmacies do the generic Sildenafil, same active ingredient as Viagra, for £5 for 4x100mg. Got to be safer than getting them off the Internet, esp if you want to pick them up quickly and not wait for the post.
The Pharmacist asks some medical questions that make sure they're not going to aggravate any pre-existing medical conditions (mostly heart conditions). You will need to confirm that you have difficulty achieving or sustaining an erection, I said sustaining is sometimes an issue, but didn't have the heart to tell the pretty young Italian Pharmacist that I needed them so I could fuck six girls for two hours solid!!
As I don't generally have ED, I find I don't need a whole 100mg tab, a quarter seems to give me a boost without too much side-effect (headaches, blurred vision & red face).