A few corrections here. The half crown (a very nice coin to receive when you were a kid - you knew your relatives really liked you when they gave you one!) was actually withdrawn in 1969, the same year as the old halfpenny. The 50p coin was introduced and the old ten-shilling note withdrawn in 1969. Decimal 5p and 10p coins were issued from 1968 in place of old one and two shilling coins, which were the same size and weight and which remained in circulation until the 5p and 10p were reduced in size in 1990/1992. The Crown (5-shilling) coin was only issued as a commemorative coin and didn't normally circulate; they continued to issue coins of this (huge) size as commemorative 25p coins until 1990, and since then commemorative coins of this size are still sometimes issued, but with a face value of £5. The decimal halfpenny was withdrawn on 31.12.1984.
20p coins were introduced in 1982, and £1 coins in 1983, with the Bank of England ceasing to issue £1 notes in 1984 (there were also large, thick £2 coins occasionally issued from this time, similar to the £1 coin, but they were commemoratives and rarely found in circulation). £2 circulating bimetallic coins were issued from 1997 (the earliest coins are dated 1996 but introduction was delayed because London Transport couldn't modify their ticket machines in time).
1p and 2p coins are the same weight and diameter as when they were first issued in 1971, but they are slightly thicker as they're now made of copper-plated steel, since fluctuations in the world price of copper occasionally made them worth twice their face value for their metal.