Setting up a UK charity affiliate

I was wondering if anybody could point me in the right direction about how to set up a UK affiliate of an existing US charity. I did look on the charity commission's website but didn't find anything specific. Would they have to through the same process as a new UK charity?

The US charity has a lot of UK donors and would like to be able to accept payment in pounds (that in itself is not a reason to set up a UK chapter) but it would be good to be able to claim gift aid and offer higher rate tax payers the incentive to donate here in the UK (I don't believe they can claim on US charitable giving).

Are there any disadvantages to setting up here?

Thanks

David