To be honest I don't know the reason as to why it's doing this, although perhaps somebody else with experience of selling items via eBay's Global Shipping Programme would be able to give you some more accurate advice on this matter. The only thing that I can think of is that some items may be ineligible to be posted via the Global Shipping Programme, which could explain why you are not seeing a postage option from the USA to the UK.
One thing that you could try if you really want the item badly enough would be to contact the seller and enquire as to whether they would be willing to end the original listing and relist it for sale on eBay again, albeit outside of the Global Shipping Programme, with an international postage option included so that you could make payment for the item. If the seller is willing to do this for you then you may well be able to purchase the item that you are currently finding it impossible to purchase via the eBay Global Shipping Programme.
