Your own domain

It is crucial for your web site to have its own domain (http://www.MyShop.co.uk). The search engines are just one reason why you should do this. Ask your provider how your domain can be linked to your shop or web site.
The link to a product could then look like this:
http://www.demosite.co.uk/ProductXYZ
The important thing for the search engines is that the link from your domain (http://www.demosite.co.uk) goes to the actual URL of the platform. This should be done technically with HTTP code 301 (moved permanently). In no case should Javascript, META refresh, frame, or splash pages be used for forwarding. These are not liked by search engines. Discuss the technical details with your provider.
The forwarding of your domain http://demosite.co.uk to the actual home page cannot contain any changes (for example, http://demosite.co.uk/epages/demosite.sf). Search engines downgrade the link even if the WWW is missing.