you forgot adam carroll!
as per what the others have said, i think the national class is a *vital* part of british f3. anyone wanting to move up to the main class the following year can familiarise themselves with the teams and make better decisions about their future, and that in turn keeps them within the same grid for longer. it's good for the main class teams because they can offer a lower cost alternative in the same environment and with the same people which is a positive thing for younger drivers, especially the foreign lads moving over here. consistency and all that. it's a similar car so they don't have to start all over again when they move up to the bigger class, and they're familiar with the circuits so they don't waste budget and time doing that learning in the main class. for the teams it means the chassis (and their data) retain value a bit longer, and can be repurposed within the same team without faffing around with resale.
it also allows the smaller, more family based teams a chance to compete alongside the bigger name teams which provides a wider range of budget options. that means there's more chances for young engineers and mechanics to get work experience on bigger, slightly more complicated single seaters. and of course, more cars for the guys to race against, and more for us to watch too
so yeah. that.