Previously, I bought all my pipe fittings (3/16-40 and 1/4-40), displacement lubricators and globe valves from PM Research in the US. I also bought some of their steam engine kits which ran very well. But that was some years ago. Recently I assembled an online order from them but when I asked for a postage quote they just gave a large estimate (table rate) which was well over and said that the difference will be refunded upon posting. I have never heard of this method when buying online before and I thought it was a bit odd.
Watching a Youtube video last night, the English model engineer said that he buys a many of his pipe fittings from PM Research in the US so I am guessing that you do not have a UK manufacturer of such items ?