Since you are using the British standard, you obviously cannot use NPT (unless you plan to convert back and forth).
The Swagelok tube fittings are proprietary fittings, and you will just need to go to (airline) tubing. This could be a workaround if you cannot/do not want to use NPT parts.
They're not proprietary - the copyright ran out years ago, and Swagelok has become the industry standard for double-ferrule compression fittings. But tube fittings do change things. Easiest to go with BSPT if you can find all the parts. Swagelok sells BSPT/NPT adapters as well.
If for some reason you do decide to play around with tube fittings, here's my work-in-progress master "Lok" list of companies that sell tube fittings functionally identical to Swagelok. The "SS" and "type/size" instances are references to stainless (and where it would appear in the model #), versus brass or other materials of construction. You can probably ignore that part. I can't promise it's 100% accurate, but these brands are definitely all clones of Swagelok tube fittings.
A-Lok (Parker) (-SS)
Tylok CBC-Lok (SS-)
Let-lok (Ham-Let) ((type)(SS)(size))
Gyrolok (Hoke) (-316)
*interchangeable with, but not identical to, Swagelok
Duolok (SSP) ((ISS)(type))
Griplok (SSP) ((ISS)(type))
*identical to Hoke Gyrolok
Superlok (SHF) (all SS)
Yor-lok (STC) (all SS)
Truelok (all SS)
Valulok (all SS)
Hi-Seal (Imperial Eastman) (-SS)
Single Ferrule brands:
I *believe* the listed single ferrule styles are all functionally identical as well, but I have no first-hand experience with them. Swagelok doesn't do single ferrule.