This is something I do from time to time although the majority of my carp gear is ordered from US online stores. If you are ordering something small and lightweight, you can get some fantastic buys from UK online stores. For instance, yesterday I ordered a spare set of bite alarms from Harris Sportsmail, http://www.harrissportsmail.com/ , a store I have dealt with before and one that I find to be very reputable. Their prices on sale items can be fantastic. I ordered a set of Wychwood alarms (yes, I am trying an alarm other than a Delkim or Fox). These alarms get rave reviews. In the US I would have paid over $40 for this alarm, an average price for a good alarm. Harris is selling them for $26 on sale right now. I bought two. Additionally, the shipping cost from the UK for both items is less than $4, a rate you can not find in the US. I am getting this product for a little more than half price. Can't beat that!
Be aware, though, of the pitfalls of ordering overseas. Make sure the store is reputable. I have only dealt with three online stores in the UK: Harris, The Tackle Box, and Specialist Tackle. Realize that it could take some time for your order to arrive (usually a few days longer than US), heavy items will cost way more to ship which will eat up any savings. And, I imagine that returns could be a nightmare (hasn't happened to me). I have had nothing but good experiences with overseas ordering at the three stores mentioned above.
Check out the Harris Sportsmail website. It's like a superstore of carp gear. You will be amazed at some of the low costs and savings.