The Reform Party went from zero seats (barely existed / did not contest elections in 2023) to over 1400 out of roughly 5,000 seats. Labour lost Wales to the Welsh nationalist party. The Scottish National Party came the closest to 50% in years (Labour only held onto 4 council seats so far). Some indications that Restore UK candidates in a few jurisdictions cost Reform even more gains.
https://www.bbc.com/news/live/c1428pev1n0t