There are a lot of moving parts with this one. On the UK side, I would recommend you consult with https://www.avaskaccounting.co.uk/ in regards to paying VAT taxes as a foreign LLC. Separately, the US LLC will have form 5472 https://www.irs.gov/pub/irs-pdf/f5472.pdf required each year, and it has a big fine if late now $25K so make sure you get with a CPA to file it on time. Hong Kong is an issue if the entity has effectively connected income in the US. or not. This is where a Tax attorney would come into play. There is no US tax treaty with Hong Kong.
I hope this helps.