Also, CAF offer a bi-mode version of their Civity regional trains (which I believe has been ordered for New South Wales, although I'm not sure if they have a more generous loading gauge than the UK) so the FLIRTs were far from the only bi-mode option available.
It's worth noting that the NSW order was diesel-only until it was converted to bi-mode in late 2019, and if my understanding is correct they haven't actually built any bi-mode Civities yet, for anybody. Stadler's product was at least proven in use on the continent, even though they hadn't ever delivered stock for the UK before.
Regarding the loading gauge; yes, there's quite a bit more room. The Xplorer/Endeavour DMU vehicles they're replacing are 25.25 metres long, 2921 mm wide, and 4110 mm high - basically a little bit smaller than UIC GB.