It is never too late to learn. We were all newbies once.
A 20 liter tank is slightly bigger than the minimal recommended tank size for keeping aquarium water under stable conditions. With adequate heater and filter, it can be used as a betta tank for just one betta fish, or a single dwarf gourami, or as a small community tank with up to 5~6 small size tropical fish of nothing more than 3" long. You can also use it as a shrimp tank for some small freshwater shrimps like red cherry or yellow shrimps.
I am not familiar with fish stores in UK since I am not from there. I randomly Googled it online and found the following tanks.
If you have Walmart in UK, it is likely they also sell fish tanks. My local super Walmart had 10 gallon tank for $14 last year. They also have 20 and 29 gallon kits. Although I have never bought a completed kit, I believe some of them offer better deals than if you buy all the stuff desperately. The problem with "compete kits" are that they might not be truly "complete". Sometimes they don't have a heater, and sometimes they don't have a filter or an air pump. You have to look in their descriptions or even have to call them to make sure what they have in the kit. The advantage with buying everything yourself is the freedom of choice. You can always pick the brand or model you like.
We were all newbies once. 
