Who was the last English/British/UK King whose first two sons (or first two legitimate sons) both had a son (or both had a legitimate son)?
Queen Victoria was the last monarch of either gender.
If we don't include sons or grandsons who are known to be illegitimate, then the last possible male example I can find is Henry III (lived 1 October 1207 – 16 November 1272, reigned 28 October 1216 – 16 November 1272), but it seems implausible that there would be no one in over 700 years.
If we include a King's illegitimate sons, then it might be George IV (I'm not sure which of his rumored or reported illegitimate sons were really his, so I can't tell who his first two illegitimate sons were).
If we include a King whose first two legitimate sons both had sons, but didn't both have legitimate sons, then it might be George III (I'm not sure if Prince Frederick had a son).
Or maybe Henry IV, whose first son (Henry V) had a legitimate son and whose second son (Thomas) had an illegitimate son.