There is a Wikipedia article that explains the advantages of mirroring (or hosting in the country you are targetting):
Amazon CloudFront is described here:
Data on UK v US download times isn’t going to be easily available because it is data that changes over time on the scale of months, weeks, days and time of day. However, it is logical that the fewer servers and Internet providers that are in the chain to the file you are downloading the more reliable and faster the download will be.
You can test this out at busy times (try 7pm in the evening) using this website:
It will show you a map of the UK, but you can drag the map and test your download speed from servers all over the world. I find my ping time is 9 times greater to San Fransisco, and download usually 20-30% slower. Even ping times can make a difference to download times because of the return time for individual data packets.
You can also see a discussion here about the issues of ping times and server locations:
Having said all that, to get all the features you want at the price you want to pay might mean you are stuck with the US hosting of files anyway. However, I hope the discussion has been interesting