As Pheonix says, we need your hardness. As your location is London, the chances are you have hard water. I've heard London water called liquid rock because it's so hard
But to make sure, look on your water company's website. You need a number and the unit of measurement rather than words. (The unit is important as UK water companies often use units we don't use in fishkeeping).
The good news is that if you do have hard to very hard water, there are many colourful fish to choose from. The main limiting factor will be the tank size; although it contains 120 litres/31 gallons, it is only 60 cm/24 inches long, being almost as tall as it is long.
In fishkeeping, it's always American gallons rather than Imperial gallons so your tank is 31 gallons not 26
As the others have said, you need to cycle your tank before you can get fish. This is the process of growing two colonies of bacteria which remove the ammonia excreted by the fish. It takes several weeks to grow these colonies though this time can be shortened by using Tetra Safe Start.
This process is what seangee is referring to and the link in Pheonix's post is the method for carrying out the cycle.
