I received the following email from Costal Contacts.co.uk today: "I had a look at your site and noticed that the information provided to you by Ellie [Ed: Ellie is a support person that I contacted yesterday] was actually inaccurate.
While we do ship from outside the UK, our prices do reflect the addition of the 19.5% for tax and duty. This is because we import the lenses to ourselves (from our local facility to our European branch) and then put the packages in the Royal mail to the customer. The customer will not have to pay the duty and taxes as we pay them on import."
This information is confirmed by this document on their site (see the shipping information: taxes section).
I will be keeping my eyes and ears open to see if the above is accurate, but to the best of my knowledge, it is, so I will remove the quote from Ellie from my post yesterday (which seemed to indicate that they did not include VAT), and include Costal in my new price list (which I'm working on now).