I've bought lots of metal and plastic airgun parts like sights, pistons, cylinders, breech blocks, springs, cocking levers etc over the last years-
just got a package with trigger guards, sights and muzzle weights from Knibbs yesterday.
I've had packages sent from Knibbs, Chambers, and several individuals on this forum and sellers the "bay", no problems.
Everything has been sent via Royal Mail, sometimes marked as airgun parts, and sometimes as cylinders or engineering parts.

Last month I got two wood stocks exported from the UK to me here in Norway, sent via Parcelforce no problems.

An issue you need to be aware of is that the payment info should not identify the purchase as gun related,
as a lot of "woke" companies create their own rules, and dont differentiate between airguns or other guns.
You are not breaking any laws by buying or shipping airgun parts from the UK, but some companies are just not gun friendly,
so descriptions like "engineering piston" or similar may be a good idea in the payment info.

The only problem I'e ever had was a Webley Vulcan without barrel sent from the UK back around 2009.
The tracking shows that it "disappeared" at the export terminal in the UK, and never left the country, but the courier didnt take any responsibility.

But as stated, individual airgun parts have never been an issue.