Depending on the legal status of your company (Sole trader, limited company, LLP, etc) you have to display various information to customers, or potential customers. This may include your Registered Company Office and / or VAT number.
The Business Link contains a very good guide for the novice entrepreneur on setting up an eCommerce page as well as other issues to consider, which can be found here:
Business Link advice on 'eCommerce'
They also provide a list of the various laws and regulations that you need to be aware of:
'The Law and Selling Online'
'Privacy and Data Protection'
As of January 2011 VAT has become applicable on some Royal Mail deliveries. This may not affect your general postage, but if a customer asks for a special type of delivery, the package is over a certain weight or goes to a certain destination, VAT may be applicable. The store options allow you to make a delivery VAT inclusive or VAT exempt but it is incumbent on you to make sure you choose the right option and charge the customer the right amount (or if offering free delivery to include it in your pricing options).
Here is a link to the Royal Mail summary page which you may find of use.
These links are provided for information only. It is your responsibility to ensure your site meets the regulations and applicable laws for the Country it is trading in. Magento, its partners, parent companies or subsidiaries, accepts no responsibility or liability for failure to heed or adhere to the law and / or regulations applying to your trading practise.