I think the reason Amazon are not selling the Fire in the UK yet is because they don't have the necessary media deals in place. Opinion seems to be that they're losing money on the actual device and look to make the profit on the media etc. that the users consume on the device. So, without that in place, it makes no sense for them to release it in the UK.
Even so, I think a lot of people will be tempted to hold out for the UK launch, so it will still probably hit sales of other tablet devices.