So what? These close the gap between what can be achieved from decent secondary regulators (about 5psi, but most secondary regulators wont hold 7 or 8psi) and what has been the maximum from variable LPG regulators (about 2psi).
2psi should hold a carbonation of about 1.1v/v CO2, which might be ideal for a "Real Ale" emulation. But I found I would go into Pubs and their "Real Ale" popular beers (bitter) would be more carbonated than that. This would be the result of inefficient (or intentional!) venting. These 20-300mbar regulators would allow emulating those "less than desirable" situations.
At the other extreme (to placate some die-hard "Real Ale" drinkers, but perhaps send some apoplectic), the higher output pressure could drive 60/40 mixed gas and create carbonation levels as low as 9psi … absolute psi that is (relative to a vacuum, not atmospheric pressure). You could do this with a breather, but would have no control of the speed of change, and breathers are "problematic" for gravity dispense and very risky with Corny kegs.
They are not easily available through "domestic" gas retailers, but are used by chick rearing establishments (for chickens or game birds). I got them here: https://gasolec.co.uk/webshop/game-m-ty ... -game-5-8/, price (about £5 more than the 50-150mb ones) includes VAT and delivery (also available here: https://ebgasservices.com/product/cless ... -20-300mb/ and other places no doubt). They come with the dreaded "POL" attachment (hose barb on other end) which are very difficult to remove (thread lock compound used). Takes a bit of effort and I suppose it would be possible to strip the threads. 7mm Allen key removes the "POL" adapter.
It is near impossible to effectively manage such low pressures without LPG regulators. Note LPG regulators must be connected to a suitable primary regulator; do not connect them directly to a CO2 cylinder.