I'm responding to this even though it's very old, because I had the same question recently.
When I was sent an item through UkrPoshta it was delivered by Royal Mail. The tracking number I was given worked on the UkrPoshta site until the item left Ukraine. Entering the same number into the Royal Mail tracking system returns "Tracking information is not available for this service" but this may just be becuase of the delivery service that the seller used.
The other thing I've learned from this is that I like Ukrainian stamp designs. I hope this information helps someone in the future.
