Uk has same type of 4g LTE network as Australia which is a different band width to USA and is not directly supported with the iPad 3 however in saying that the performance at 3G on the 4 g network is impressive when comparing. a iPad 2 on 3G
From what I can understand it is the design of the antennas in the iPad 3 that makes it faster
One should understand that LTE 4g may only have a small range in cities so the iPad will drop down to 3 g outside of the 4g transmission iIn Australia while traveling I found if you are out side 4k of a major city forget it. In some cases less than 2 k I tested the LTE network with a iPhone 5 which is. a special model for our region I suspect the latest 4 g iPad and the mini just released will be a special model like the iPhone 5
There are 3 versions of the iPhone 5 world wide
Personally I would. not stress to much as it could take years before the coverage is beneficial to you.
The real problem is not Apple it is the fact that there is no single standard LTE 4G network across the world so if you travel a lot regardless what you have it may drop down to 3G out side of your country
Hope that helps