Here's how Development responded when I brought this matter to their attention:
S3 is not a CDN; when they download from it, it comes from the particular S3 site we've uploaded those files to (which currently is going to be an Amazon data center in northern VA). I've done some preliminary work (i.e. unfinished) in adding a CDN layer (CloudFront specifically) to that process, but there's a cost issue there such that we can't just throw
downloads through CF without running up our bandwidth costs; if we do it selectively and correctly, it could actually lower our bandwidth costs. But it's a major project and it seems like there's always something more critical in the big picture of all our internal and our sellers' wants/needs.
As for getting better performance from his server in Utah, that's all about the particular peering details between different ISPs. It very well could be the affected UK ISPs are throttling downloads from whatever peer network the s3 traffic ends up coming from because it costs them more... the situation may very well be reversed from other locations.
In summary, we're aware that reliable and fast connectivity to our particular S3 server farm can be an issue for certain ISPs depending on which upstream networks they use to connect to the Internet at large, but the remedy for that is a major and cost-sensitive undertaking, while other projects of broader impact to most/all buyers and sellers using our service have taken precedence lately.
One thing to bear in mind about providing direct links to your server is that unscrupulous buyers could share that link URL with others who could then download directly from that URL indefinitely, whereas we issue each buyer a unique link that expires after the max. Attempts or Hours you define in the product's settings. Rather than providing that fallback link to all buyers, you might simply offer an invitation to contact you in case of download issues (with a link to your email address or contact page), then provide your server link to those buyers personally. If you'd prefer to continue providing fallback links to everyone, it would at least be a good idea to change that file's location/URL every so often, so any old fallback links floating around out there would stop working.