There will be a very large fuse located before the Electricity meter. The seal must be broken and the fuse pulled out. This will isolate the meter and the consumer unit.
Before doing this, however, you must contact your Local Authority Building Control department (LABC) (look in your local Yellow Pages for their number). You will need to apply for permission to do the work. If you believe that you are competent to do the work yourself, the law allows you to do so, but it must be inspected and tested in accordance with the requirements of the LABC. You will find that they are very helpful and will advise you on what you need to do.
They will charge you a fee when you make the application, but will, at their expense, appoint a qualified electrician to inspect your work at least twice.
But, above all, [b]the law now requires that you notify this type of work[/b] to the LABC.