I would say Bft 5 as a mean, but sometimes certainly 6.
Wind is not the biggest problem as you will know. The waves and especially the surf was unbeatable. Looked sometimes as a tsunami. You cannot pass a massive wall of water of perhaps 2 meters high that is rolling over you as a steamwalls. You can only pass it if by accident your timing is right and you don't collide into another boat that is without skipper or that is pushed on you upside down. I can tell you that I saw about the same scenery last year but not so bad. At that time I sometimes could laugh about the slapstick scenes you could see. But this was different, I felt sadness about all those damage I saw. And sometimes I felt a little bit mad when I saw people that had clearly con clue what they were doing. For instance a Dart with 2 rudders in upside position and two people that didnot understood why the boat was turning into the wind every time and why it just didnot sail into the sea and through the waves. And so on. It was a little bit shocking to have all those scenes before you and every ten seconds something else dreadful happened. I wonder when the first human casualties happen. This round was only for the very experienced in sailing through the surf of the North Sea and also with a lot of luck. Mitch Booth even crashed. Experience was not enough.