SCO19 Spotted With MCX Carbine
Recent pictures taken at a football match at Wembley Stadium showed members of SCO19 carrying a new carbine. 
The elite Met firearms unit has been expanding in size and capability as part of an ongoing strategy to prepare for an incident such as the recent terror attacks in Paris. SCO19 has increased the number of Counter Terrorist Specialist Firearms Officers (CTSFO) and beefed up their firepower. New weapons include the Sig Sauer SG516 rifle and now, the Sig Sauer MCX carbine.
The MCX is a fairly new firearms product, being introduced on to the market in early 2015. It works on a gas operated, short stroke piston system and is available chambered in .300 blackout, 5.56x45mm NATO and 7.62x39mm. SCO19 have apparently opted for the 5.56x45mm variant. The MCX accepts standard 3-round STANAG magazines.
The MCX is designed to be a flexible and modular system that can be configured for a range of missions. It's thought that SCO19 typically run the MCX with a safe-semi trigger group, 11.5 inch barrel and telescoping stock. The MCX features rails for mounting scopes, firegrips and lights. A mini red dot sight augmented by a flip-to-side magnifying scope have been seen in use by SCO19 CTSFOs.
In September of this year, SCO19 officers operating in a dignitary protection / counter assault role were spotted in a Land Rover Discovery. Photos in the papers showed that the officers were armed with MCX carbines. The short length of an MCX with stock collapsed makes sense for operations in tight confines such as working in vehicles. 
More info / further reading
- 1. Chilling new face of police in Britain: Female 'robocops' dressed in military fatigues and armed with semi-automatic rifles are new face of counter-terrorism
(The Daily Mail - note: the article identifies the MCX as the SG516)
- 2. Don't mess with the Met! The terrifying machinegun-toting anti-terror police who are guarding Israeli PM Benjamin Netanyahu during his London visit
(The Daily Mail - note: the article again misidentifies the MCX, this time as a G36)