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Land registry is the original source, Zoopla, Rightmove, etc (I found Zoopla's sold listings better than Rightmove's, can't remember why) get them from there. It fell quite a bit behind during covid and there still seems to be a fair lag now (although it does seem to have February transactions in it).
Main benefit of the Land Registry site is that you can download a load of info (I did all transactions for the first half of the postcode) and then play with it in Excel to get the info you want and look up specific properties on Zoopla or whatever.
Data can be downloaded here. It's free to use and download
http://landregistry.data.gov.uk/app/ppd/
I reckon when I was selling/buying I had way more idea of actual prices than the estate agent. They seemed amazed when I said I'd downloaded all the sold prices from the land registry.