Here you go http://bitcoinmagazine.co.uk/subscribe/

I do not think you really need to change your existing subscription/order if you just want to buy more. You could have as many of the options listed on that page as you like. However, support@bitcoinmagazine.co.uk is where customers can get support when needed.

Since some found language on order page confusing we will review it and try to make things more clear. Thank You.