I am in the second round of racing there next month also. A year or so ago, Thruxton was my very first race and I had a ramp test FTP of 225 then, so actual FTP was probably a little lower. Maybe 210w. I stuck with the bunch the whole race and didn’t get dropped until the little uphill with the chicane at the end. Obviously things may be a bit different this time with Cat 4 clogged up with people that should be 3’s or even 2’s.
Try and stay in the first 10-20 riders and see what happens 