Amazon in the US tends to do the investment in R&D. How much does it do in the UK? From the sounds of it not much. It doesn't matter how much R&D it does in the US Amazon UK can't write it off.
Sure it can. They can license the results of the R&D and write off the license fees to Amazon US. In fact, the US tax authorities might even demand that Amazon US collect license fees from foreign subsidiaries - otherwise they would actually transfer profits (the arm's length license fees) from the US to foreign countries.