You can get one straight away as I understand it. The website showing August, however that is probably a short term indicator, as last week, the M3 delivery time was March. So I doubt that Tesla are skipping an entire quarter of deliveries to UK, delivering in March and then skipping Q2 and restarting Q3. I don't think they've done that before.
In any case, a chap on another forum just ordered one of those August delivery Model 3's, and Tesla came back with a March delivery. I think once all the madness of Q1 deliveries gets sorted out, which will no doubt be much higher than normal due to MY, the website will update again with proper dates you would like to think.