British

British QF 1-Pounder Mk II AA gun

Knut Erik Hagen took these photos at IWM London in July 2003.

British 9.2-inch gun

Knut Erik Hagen took these photos at IWM Duxford in July 2003.

9.2 Inch gun from Gibraltar which has been modernized many times, it did at least start out as On-Topic.

British 4.5-inch Howitzer

Lance Krieg took these photos at IWM Duxford.

British 9.2-inch Howitzer

Australian War Memorial

Ross Moorhouse took these photos in 2005.

IWM Duxford

Knut Erik Hagen took these photos in July 2003.

British M-1886 6-inch Howitzer

Knut Erik Hagen took these photos at IWM Duxford in July 2003.

British Mk IV Naval Gun

Knut Erik Hagen took these photos at IWM London in July 2003.

Museum plaque seems to read "British x Inch (or x Pound) QF Mark IV Naval Gun".
Knut: I believe is from HMS Chester and one of the gun crew became the youngest recipient of a VC.

British 13 Pounder

Lance Krieg took these photos at IWM Duxford.

Robert Horton: Could be an 18-pounder

British 60 Pounder

Knut Erik Hagen took these photos at IWM London in July 2003.

Stokes Trench Mortar - 3 inch

Australian War Memorial

Ross Moorhouse took these pohotos in 2005.