I've tested Smarty and ID mobile on Three, Lebara and Talkmobile on Vodafone, and 1pmobile on EE. They all work fine, as long the network is good in the area.
Some points to be aware of:
- Only Three offers a fixed IP was far as I'm aware. Everyone else (even Three's virtual networks/MVNOs) uses CGNAT. Some (EE) give you an IPV6, but V4 is CGNAT.
- Some EE MVNOs don't have access to every band. Plusnet, RWG, Lyca, etc, don't have access to band 20. Not that it helps a lot with speeds, but that's all that you get in some areas.
- Some users on Vodafone and their MVNOs have been reporting high packet loss for some time. It doesn't affect everyone or every part of the country, but it's something to test.
- Check the fair use policy. Lyca recently moved from O2 to EE and introduced lower limits. The last unlimited plan I've checked from them had a 400GB limit, which is not enough for everyone.
- The cheapest way to get unlimited on EE is probably with a Scancom SIM (check on Amazon for best prices). You have to pay upfront, but it costs less than £15 a month. By the way, EE has a 650GB limit, but they don't stop the service, you just get de-prioritised until it resets (still fast enough in most areas).
- You often can find cheaper plans on sites like uswitch or Money Saving Expert, might be useful if you want to order a few SIMs to test all networks.
- When choosing between O2 and Vodafone, it's useful to look at this map. They share masts and with their arrangement the host network usually offers better speeds in the area (eg: O2 is better in Canterbury while Vodafone is better in Bournemouth):
