No complaints about 1pmobile. They don't have the best looking website or even a mobile app, but with 5G, B20, wi-fi calling, volte, etc, it's a nice option if you want to use EE without paying EE prices.
PopIt... never heard of them and their plans are not as good as 1pmobile, but they might be a good option for roaming:
We do not cap the data usage. If you have 1GB,3GB,10GB,30GB or 100GB you can use all of this allowance in the EU.
I wonder if there's any hidden limit. Speed caps after xxGB or something like that.
Smarty.co.uk is a nice option too, but they use Three which is not good everywhere.