If anyone still querying this . . .
Bought one of these 3 days ago. On the phone today to Garmin GB support - no, the inreach mini cannot be set to display GB grid refs and therefore cannot be used in the UK for location fixing or navigation to a waymark. Extraordinary. My best coordinates/datum option on the device was UTM/UPS & WGS84 which, natch, bears no relation to os maps. Device returned and Garmin Billing were quick to reply by email and decent enough to refund my monthly subscription of £14.99 and the (surprise additional, grrr) activation fee of £24.99. Ludicrous to market the mini over here until it can be used with a paper map & compass - which anyone setting out should have and know how to use. Even the Ordnance Survey online shop is flogging these.
The Explorer is the next level device and has a high profile everywhere online but I see there's also a post elsewhere on this forum asking if it can used with GB gridrefs.
I think you're probably going to find that the first iR device to support this is the GPSMAP 66i. This is the first model which is a full member of the Garmin ecosystem. (All prior models have their roots in the original DeLorme products. The Mini is sort of half and half, but obviously still has very limited choice of datum.) The 66i inherited its GPS functionality from the standard Garmin handhelds in the 66 line.
I have no way of verifying that it works (I'm in the central US) but Ord Srvy GB IS an available datum on the 66i. When selected, it uses the Airy spheroid. (I have no idea if that's the correct one.)
I can get Ord srvy GB Grid on my Instinct watch, so does seem a bit odd that it's not on the Inreach Mini. That being said, the SOS beacon function of the mini is my main purpose behind it & the two way texting. Do the Mini's 'big brothers' have UK Grid system?