I bet your seller didn't state that they were in the UK. They may have stated that the item was in the UK, which is different. Look at their feedback page to see where they are. Many sellers in China have a distribution base in the UK, which entitles them to list items as UK stock.
If your nail polish was listed as UK stock it should be delivered from the UK, but as you've accepted a 27 day delivery you can't do anythng until that time has expired.
It isn't just about delivery times. Items bought direct from sellers in China may prove to be poor quality, unbranded or unknown brand names, or counterfeit, and may even be dangerous. Any statutory consumer protection will be impossible to enforce, even if the item itself is in the UK.
