I've ordered 3 boxes now with Heat and every single one of them has been amazing. Now you might not get exactly what you want in the boxes, but in general each box has something that is extremely good and worth the price.
For me, I use them more as a personal shopper, to refresh my wardrobe. There's just too many choices out there and HEAT picks you something HOT that you can be more bold about with your style.
If you're unsure, I'd recommend just getting the £300 box to trial it out. There'll definitely be something decent, but if not, you can always return it or sell it on vinted/ebay.