A nice tank for a Betta is a Aquastart 320 which is a 6 UK gallon tank.This has a light and a undergravel filter (some like U.G.F ,I dont so i replaced mine).These are about 12" cube in size and are glass so it wont scratch easily compared to the Argos plastic thing and are available in most fish stores (or second-hand on E-bay).
You can get them from most places new for £40 or for around £10-25 second hand from E-bay.
As for quality,comparing one to the Argos tank is like comparing a Aston Martin to a Robin Reliant
.(I have one for my Betta & shrimp tank,look at my aquariums on my avatar).
For Betta food they do sell Hikari Betta Pellets in a lot of fish shops (including pets@home I think).
P.S. here is a link to a uk site that shows the tank I'm talking about
Aqua One Aquarium Aquastart 320 Silver 28l
Just noticed that the Argos tank doesnt seem to have a light,this can be a pain in the bum too!