The two main UK federations are IPF (tested) and GPC (non-tested). You can find out their fixtures from their website.
Each federation has regional committees, so will depend on where you live, plus there's smaller local feds and friendly meets.
There's also BDFPA (British Drug Free Powerlifting Association) which have quite a few local meets.
Look on Facebook, lots of smaller feds use their FB pages to post meets, etc.
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