Wiggle delivers free to the US on orders over $60 USD; I throw tubes in with other orders when my supply on hand is below, say, a half dozen. If don't know about their policies to the UK or EU markets.
Even with $8 in shipping, if you buy 8 tubes that only adds $1/tube. Usually still well below shop prices, which can be as high as $7-8/tube (although less obviously so at $2.80 than their former price of $1.50).
> I also believe you are wrong if you think that sourcing internationally is anywhere within the "low impact" range.
I guess we have different ideas of what that vague term means, and that's fine! For what it's worth, the entire bike part ecosystem is international. The vast majority of bike frames are made in China and Taiwan; Shimano's parts are made in China, Malaysia, and Singapore; and very few brands (Look; I don't know of any others) make frames in North Africa, which is relatively local to the EU but not the US market.
Inner tubes are also made in Taiwan/China. Unless you live in Taiwan, there is international sourcing going on somewhere in the supply chain.
Happy cycling :-)