Hmm. All depends on whether you count the Crown Estates as King Charles IIIs land as if it were his private land.
#1 Forestry Commission with 2,200,000 acres
#2 Ministry of Defence with 1,101,851 acres
#3 Crown Estate with 678,420 acres
(Which while owned by King Charles as monarch, it isn’t privately owned by him and he only gets 25% of the profits from its revenues, the rest of the profits go to HMRC Treasury)
#4 National Trust and National Trust for Scotland with 589,748 acres
#5 RSPB with 332,000 acres
Finally, our first private owners start appearing….
#6 RICHARD SCOTT, DUKE OF BUCCLEUCH & QUEENSBERRY 280,000 acres
#7 ANDERS HOLCH POVLSEN (Danish billionaire ) 218,364 acres
#8 THE DUKE OF ATHOLL’S TRUSTS 145,000 acres
#9 UNITED UTILITIES (dam energy companies) 140,124 acres
#10 HUGH GROSVENOR, 7TH DUKE OF WESTMINSTER 140,000 acres
#11 DUCHY OF CORNWALL, 135,000 acres
(similar to Crown Estate, not privately owned by the heir to the Crown)
—— lots of other people —
#35 QUEEN ELIZABETH II, 20,000 acres
abcfinance.co.uk/blog/who-owns-the-uk/