It's weird indeed, the gun SHOULD have been able to take 144a as it's pretty much the weakest gas you can buy and a direct replacement for 134a which is what Marui recommend.
UK retailers have no contact with TM as a company, they buy all their stuff through Japanese wholesalers as TM do not export. The only place that realistically would have been able to help you was
http://pulse-japan.jp/index.php?sl=EN unfortunately they're closing down (which is a real worry for me, since I use them to get a lot of TM spares!) so you're probably pretty stuffed for getting a replacement plastic slide new. Would be worth putting up a wanted ad on a few forums though, someone out there must have swapped out their plastic slide for a metal one and be willing to sell you their spare