I looked into this and found nothing other than the statement in the handbook that the RX is not designed for off road driving.
Personally I wouldn't drive through water that could potentially rise above the door sill
Megawatt Charging System will kill hydrogen trucks once and for all, says electric truck expert
https://chargedevs.com/newswire/megawatt-charging-system-will-kill-hydrogen-trucks-once-and-for-all-says-electric-truck-expert/
I'd be surprised if such a safety related warning also referred to anything else. Imagine driving down the motorway and the light came on. Do you stop when safe to do so and check your tyres? Or do you continue 'cos it might be "something else"?
If you have an accident involving a third party you must inform your insurer (it's in the T&Cs you agreed to) because should the third party claim on their insurance your insurers will find out as all companies share this information.
You should have 3yrs free updates with a new car, so basically you have approx 18 months left.
Download the multimedia handbook and all is detailed therein.
I'd recommend not using the OTA update as this can kill the speed camera alerts. Instead download to a USB instead.
Had them on my first RC and have them on my RX, well worth it for keeping the majority of muck off the sides and rear of the car.
As the protection pack is a post-manufactured addition, it's not too late...
Have you downloaded the Lexus link app and created an account?
Perhaps you have done this on the Lexus portal? Have a read of the info in this link
https://www.lexus.co.uk/owners/about-my-lexus
Then if you haven't already downloaded a copy of the mutimedia handbook do so from the website
https://www.lexus.co.uk/owners/about-my-lexus/manuals
I think there are numerous councils using hydrogen powered buses in the UK
https://www.gov.uk/government/news/uk-on-track-to-reach-4000-zero-emission-bus-pledge-with-200-million-boost
On our jaunts down to Devon and Cornwall (OK not 1300 miles but a good 250 non-stop) in the RX450h we are seeing around 35mpg. As you, I tend to stick around the speed limits and the RX is superbly designed to just cruise along, stress free.
Clearly states fully comprehensive cover on their website
https://www.dayinsure.com/guides/temporary-european-car-insurance/?gclid=Cj0KCQjwpPKiBhDvARIsACn-gzBH6lvgo5RfXBuN8M3HM7ZZc56FPWZmC_hGX1nWEAZP1wv7EPa0DjUaAmZBEALw_wcB