Got the core Starter set but it seems almost impossible to get hold of many of the expansion kits , especially in the UK. How come stock levels are so low?

They're on a boat, crossing the Pacific Ocean from the printer. FFG is still supporting the game. It's just that demand was huge.

Don't mean to sound disdainful at all but a little search and/or scroll would have answered this. But to help out...

Huge supply and demand trouble. They can't make them fast enough to meet demand. Amazon on this side of the pond currently has everything in stock:

http://community.fantasyflightgames.com/index.php?/topic/92847-wave-in-stock-and-ready-to-ship/

Looks like Amazon UK has wave 2 and 3 in stock:

http://www.amazon.co.uk/gp/aw/s/ref=is_s_ss_i_0_10?k=x-wing+expansion&sprefix=X-wing+exp

Wave 1 restock is coming. Very soon.

I spoke to a games store in the UK and they have been told it's going to be December at least before they see any re-stocks.

Hand painted ships... hard to crank them out fast enough to meet the incredible demand.