This isn't the case if you register through Companies House - you have to allocate all share capital.
I found this out when using their online registration service recently - according to my accountant it's a change from the Companies Act 2006.
The Companies House registration service is definitely worth using - you'll probably get your company set up within an hour or two of filing the form. Unless you have a complicated shareholder agreement (in which case you really should get yourself a good lawyer if you don't already have one) the default Articles of Association will be fine for you.