Ordnance Quick Firing (OQF) 17 pounder.
The best Allied anti-tank gun of WW2 and on a par with the German 75mm/L70 (the gun in the Panther).
The towed versions were crewed by the Royal Artillery, with the gun also being mounted on a variety of armoured vehicles including the Sherman tank to make the Sherman Firefly, the M10 Tank Destroyer (as the Achilles) and the Valentine tank chassis to make the Archer SPG.
Interestingly, the Archer's fixed gun pointed backwards meaning that it could bugger off rather quickly once it'd done its job. After WW2 it continued to be used in the Centurion MBT until it was eventually replaced by the 20 pdr.
All you will probably want to know about the 17 pdr is on this wiki page