Asda will only sell you Sildenafil without a prescription. It seems to either be Asda, Dr Fox, or Lloyds that people are using; there was actually a topic with a discount code posted recently too for Lloyds
https://www.ukpunting.com/index.php?topic=108286.0In terms of differences, I would say the biggest that nobody has mentioned so far is Viagra is out of patent so can be had very cheaply on generic form. Whereas Cialis is not out of patent until November 2017 so to buy it you are talking ~£30 for 4x20mg tablets.
Its crazy paying that price so buying Sildenafil is the logical choice.
If you can find them, which its not easy to do, but the best in my opinion is the Sildenafil/Kamagra Jelly Packs these remove a lot of the side effects of Sildenafil and also work super quick so its best of all worlds. The only downside of them is they cost about four times the cost of a tablet.