300 - 500mg is a huge dose, 100mg should be sufficient to blow most peoples heads off. Problem is there is no testing and you cant weigh the thc content so most people just lie about the Mg's.
Imo if you are having to eat £20 worth of edibles to get high you need to find a new vendor! We need reasonably priced strong edibles on here. The market on Insta is probably better than here for edibles tbh.