I’ve not done the Highland ones but have raced at Carlisle, Roundhey and Mildenhall
you need a track bike with good clearances, cx tyres are OK but there is a blackmarket in old Dugast diamond tread tubs. A BC racing licence is also required (you may be able to get a day licence)
unless you are really fast you’ll lose, (in Highland grass track especially if you have an English accent, if you manage to win you will find the prize money suddenly goes to the lower placed riders) grass track is a particular skill and staying upright in a bunch bouncing around a corner just as the drizzle starts requires more skill than you think
however there usually are handicap events which mix it up a bit
personally I think it’s great, it’s like 4X for track riders, its bonkers and a nice way to spend the day with the family, races tend to be short, take plenty of sprockets etc (the old adage is to start low and then go lower)