If a technology was initially invented, manufactured and sold in the UK around 2008, but not patented, and a similar technology was later patented in the US in 2015 by a different company, can the original UK technology continue to be manufactured in the UK and safely sold in the US now?
The answer is probably yes, but not guaranteed. Since the product had been sold in 2008, it probably predates the filing of the 2015 US patent. If you had listed the US patent number we could determine the filing date and the patent's status. Since the sale of the product is public it represents prior art to the 2015 patent so that patent shouldn't be a problem. What you don't know is if there is another previous US patent that predates the original sale in the UK that controls the invention in some way. Just because the one patent you found isn't a problem doesn't mean there aren't other patents that are. I should point out that since you are manufacturing in the UK, then UK patents are also potentially an issue for infringement.
The best course of action is to do a formal freedom to operate analysis with a competent attorney. The reason I suggest the use of a patent attorney or agent is that it is what is in the claims that matter. Many people believe a patent cover much more than it actually does because they focus on the body of the patent. Indeed many questions here at Ask Patents confuse patent applications with actual granted patents. Applications sometimes don't get granted and even if they do, often have their claims narrowed before becoming a patent.
In any case, I am not a lawyer myself and this shouldn't be taken as legal advice. It is always wise to consult with an attorney in patent matters.