There is a newly passed amendment to the UK Companies Act means that any UK company has now, by law, to include the following information in their email footers (this means general email not e-mail marketing although best practice would suggest inclusion):
Your company name
Your registration number and registered address (if registered)*
Your place of registration (e.g. England and Wales)
*it is not sufficient to just include a vaild address, it must be your registered address.