It seems to be becoming good practice to install a single pole MCB after the meter to isolate the supply. This prevents you having to pull the main fuse, and protects the electricity suppliers main fuse from blowing (the single MCB should blow first.
The other (much more common) scenario is that some installations have an RCD trip-switch after the meter which protects the whole installation from earth faults. This is most common in houses with overhead supplies as the earth connection is not as good as in underground supplies, but it's good practice in any installations. This, if wired before the consumer unit(s) can be used to isolate the unit. It is double pole and is usually grey, with a test switch on it.
GEES - if that photo is of your own setup, please be aware that the black device with the yellow button, which I assume is the switch you are referring to, should be changed. It is actually an old version of an RCD (it is a voltage-operated trip rather than current operated). They can no longer be tested and are not as reliable and safe as the more modern versions. Back when I was on the tools (I worked for the regional electricity supplier), we were not permitted to work on installations with these unless the customer agreed to change it, because we could not guarantee that it was safe. A new device would cost about 40 quid plus installation. I strongly recommend you change it, especially as it appears you have an overhead supply, where the main RCD is doing an important job. These trips are no longer permissible within the regulations.
In case anyone's wondering how I can tell, and want to check thiers, the old voltage trips are nearly always black, with a yellow test button, and usually say voltage operated on them. The newer RCDs are usually grey, with a grey button, and say RCCD or residual current circuit breaker or something like that on them. If you have a voltage trip, change it!
A bit of a reference to the voltage trip issue here
http://www.tlc-direct.co.uk/Book/5.9.4.htm - good site for buying gear too!
Best wishes
Paul