Hi Ollie, and welcome!

It's been a while since I've tried to fit UNF nuts into wheels, and I've never tried it with those particular wheels, but I don't think they will fit as they are. You may need to either heat up the UNF nut with something substantial, like a blowtorch, and press them in (with the heat melting the plastic around the nut, and which will harden when it cools) or drill out the hole in the middle of the wheel to a more suitable diameter and then press (via a hammer) the nuts in that way. You should be able to do this second method without having to heat the nuts up if you get the hole size correct.

Can't offer much more than that I'm afraid. At about £2.50 a wheel it might be worth buying a few spares though just incase the first one or two attempts go slightly skewed (don't worry if they do, all part of the learning process )