Actually a Prototype Challenger I.
It is interesting to note that the British Chobham reactive armour introduced on the Challenger is also supplied to the US for use on their M1 Abrams MBT. Also worthy of note is that the Challenger holds the record for the longest confirmed tank-on-tank kill in military history, destroying an Iraqi T55 tank at a distance of 3.2 miles (5.1 km) with a 120mm APFSDS 'Charm 1' depleted uranium round.
The FV4034 Challenger II has now superseded the Challenger.
N.B. APFSDS = Armour Piercing Fin Stabilised Discarding Sabot, basically a dart.