It is actually an offence to break the seal on the Main Cut Out. If found out then the DNO has every right to take you to the cleaners! Contacting the Electricity Supplier is no good as they only sell the electricity to you! The best thing to do is ask for a main switch to be installed, many of the DNOs (distribution network operators) will do it for you for a small fee, this'll allow you to disconnect and then reconnect the supply at will. Their number is usually found on the meter. If you're not sure then follow this link to find who your local DNO is
http://www.energywatch.org.uk/your_questions/i ndex.asp
One other point to consider :- Under the New Part P regulations, the changing of a consumer unit constitutes notifiable work. You must therefore inform you local planning department of your intentions to carry out electrical work in your house. They will then carry out an inspection of work (or an agent acting on their behalf). If the installation meets the current regulations then a certificate will be issued. Oh forgot to mention none of this is going to be cheap!