I had the same £10 unlimited plan with three too, barely used it, just kept it as a spare and for the roaming, now that the new plans don’t include roaming I’ll probably just keep a smarty sim and top it up when needed.
Three used to limit the 4G frequencies MVNOs had access to , I’m not sure if they still do but smarty which is owned by 3 has all the 4g frequencies, so bear that in mind if you go ID mobile or another that runs off three.
Smarty has a black Friday offer for unlimited @ £17 now, no commitment.
(One off £12 cashback on quidco too)