20th Century Halfpennies

Edward VII (1902-1910)
Like the penny, 1902 has both low tide and high tide varieties. About 0.7-1.5% of the issue are low tide.

George VI (1911-1936)
In 1911, there are minor varieties on the obverse. The hollow neck variety is unique to 1911. The flat neck also appears on 1911, and is found on all subsequent issues with the first effigy.
1925 had two distinct obverses, but they are hard to tell apart. The engraver's initials tell you which is which. The first effigy has the initials B.M. while the modified effigy is BM. The reverses of the two are also different. The newer reverse generally is better struck and has a thicker rim.
Starting in 1928, the head was smaller, presumably done to combat "ghosting."

George VI (1937-1952)
Three minor varieties of 1940 exist., determined by "pointings" on the reverse. None are rare.
Starting in 1949, like with all other denominations, "IND:IMP." was dropped (India achieved independence). With the bronze pieces, "FIDEI:DEF" replaced "F:D".

Elizabeth II (1953-1970)
There are many minor varieties. Among the interesting ones:
1953: cross points between two beads (original "plastic set") or to a bead (standard circulated pieces).
1957: both "calm sea" and "rough sea" dies exist. The calm sea is almost scarce.
Minor varieties exist on 1954 (2), 1956 (4), 1958 (3), 1967 (2) and the 1970 proof (2).