My understanding is you need an active alarmed radiation monitor above FL490 in planet EASA, which precludes european G550's going up to FL510 (G550 AFM ceiling). I am happy to cruise as high as possible, FL470 is very achievable flying within europe, gets you more direct routing, less fuel burn etc.
The back up plan is the 'emergency descent mode' which gets you down to FL150 rapido - however! 'time of useful consciousness' (TUC) up there is only a few seconds based on age/smoker/fitness etc, so in my case it's 2 seconds.....
