If you have played on UKCS before, and you are ranked on the server you may be able to get your SteamID from the following pages:
UKCS.net SteamID find (by player name)
Team Fortress 2 Stats.
CounterStrike: Source Stats.
Day of Defeat: Source Stats.
CounterStrike 1.6 Stats.
Failing that, there are comprehensive instructions below.

Finding your SteamID:
- Load up any Steam game with multiplayer functionality (ie, CounterStrike: Source, Team Fortress 2).
- Enable Developer console (see below).
- Connect to any online server.
- Open console using the "`" key (usually below the "Esc" key.), although it may be different on your specific keyboard. "~" is another likely key.
- Once the console is open, type "status" (without the quotation marks) and press enter.
- You should now have information like this in your console (it will vary depending on the game you are currently in. The screenshot shown is for Left4Dead.):
Note the blurring is as this was taken on a public server.
The only piece of information that you need is in the format "STEAM_X:X:XXXXXXX". Copy that information and paste it into your profile. You will be able to finish registration and take advantage of in game Private Messages and the UKCS In-Game Spray Inspection script.
Enabling the Developer console in Team Fortress 2, Counterstrike: Source, Day of Defeat: Source.:
- Select "Options" from the main menu. On the first pane (Keyboard), select "Advanced" at the bottom of the window. Ensure that "Enable developer console" is ticked.
Note that this screenshot is purely for Team Fortress 2, but the method is the same for all other games. - You will now be able to use the console.
Enabling the Developer console in Left4Dead:
- Load up L4D.
- Click on "Options" then "Keyboard and Mouse".
- On the "Keyboard and Mouse options", scroll down to "Enable Developer Console" and click. Select "Enable".
- You will now be able to use the console.
Thanks to Jihn, Little_Devil, UK_Sniper, Nelsaidi2 and Trev for their assistance and suggestions.