If it is your first rodeo, it pays to ask some questions about bulls. Or in this case, you seem to keep having problems because you don't know about shipping rules, but you keep asking strangers on the internet
to ship things to you. Across international borders.
You can't assume folks know what they are doing, just because they have a web site. Odds are the UK chandlery
you contacted bought a cheap
"online store" web package, and the package doesn't take hazmat into account. We had the same problem in the US 10-15 years ago, when many web commerce packages could not deal with simple shipping weights and charges or sale
tax venues. Fifty states and many of them have 50-100 different sales tax rates varying by county and/or postal code...plenty of room for problems.
Hell, we had truck dispatch and loading software
routing trucks onto restricted access (no commercial
traffic) highways and making multiple toll crossings without accounting for the toll costs. Just because someone sticks something on a computer, doesn't mean it works right. Garmin
and Google Earth
still get things wrong, and Apple Maps...well, they've set a new standard for hoof in mouth.
Usually when you go looking for the cheapest source, you're going to find people who just don't have experience. And then if you are very lucky, they will still step up and do the right thing. More often, they simply drop the ball.
Any chandler in the UK could ask Seago (who are UK-domestic) to supply a raft without the gas bottle and flares. Or, they could volunteer to remove them, although since they are not repack agents that would void your warranty so they might not want to get involved in that. But you could also ask Seago about supplying a raft without the gas and flares, with an empty gas bottle supplied.
Of course, there are apparently no authorized repack agents outside the UK, so even if you got what you wish for, you might find a problem three year from now: No one else will repack it.