Any tips for my first time fixed gear crit?

Fixed 42 isn't like other fixed gear races because it's on big open roads and there's no sharp corners. You want to put on the biggest gear you can push at about 100rpm for an hour because it's fast. That said, there's literally hundreds of people and a good chunk of them have never raced before, this means there will be dodgy moves, people overlapping wheels, moving sideways without looking etc as well as some that are just nervous.

Do some crits if you can and get comfortable riding in a big group, taking corners shoulder to shoulder with someone, learn how to move up through a group without causing a crash etc. It all comes with experience, the more you do it, the more you'll learn.

NL crit series is meant to be very well run, there is a strong field and good courses, it will be a good introduction to fixed crits. Thundercrit will of course be awesome, the circuit is good, the field isn't too massive and there's split groups of abilities so you don't have to race against the fastest guys if you're just starting out. I'm not sure when it will be but Minet Crit at Hillingdon is good too.

You're on this forum so I assume you're comfortable riding fixed around traffic, but if you've not ridden brakeless before then give it a go, obviously not on busy roads at first, but practice how fast you can take corners, how low you can lean and how to emergency stop if you need to.