I don't think you are supposed to and will have to deduct it post purchase. You could always ask your regional support or handle it through a reseller. These things vary too specifically froim region to region, but Adobe does have a pretty large UK branch, so I'm sure there's someone you can talk to...
Mylenium is correct you will want to work with our sales department and provide them with your registration number. There is still an approval process to go through though before you will be enabled to make purchases without the VAT applied. You also will need to purchase the download version of the product. It is a requirement that we apply VAT for any physical goods which are purchased. You can find more detals at Adobe Stores | Online order and payment FAQ - http://helpx.adobe.com/x-productkb/policy-pricing/stores-online-order-payment-faq.html. I would recommend reviewing the Taxes and VAT charges section of the document.