I have spent some time optimising my website www.wetsuitcentre.co.uk using results and guidelines from pingdom.com which initially showed an improvement.
Some pages remain stubbornly slow though - over 10 seconds in some cases
After a very quick glance I’d say your shop-system takes a very long time to piece the page together before delivery: the browser is waiting for a response from your server for ~7 seconds.
Many online-shopping-systems have performance problems depending on what you throw at them - I’ve seen this happing to OXID Shop as well as XT-Commerce.
If your shopping-system is indeed based on PHP, an obcode cache like APC might help you improve server-side performance a lot. Additionally, trying to cache pages for viewers with Zero Items in their shopping-baskets (stateless) with e.g. Varnish will help a great deal. Try to keep your site-visitors stateless as long as possible so that they can all be server from Varnish rather than your actual webserver (NGINX, Apache…).