Why are British cooperating with Americans to help al Qaeda in Syria?

This is the big story of the day on the war front, out of London:

Resurgent al-Qaeda plots deadly attacks against UK and Europe

By Deborah Haynes, Defence Editor; Hugh Tomlinson, Delhi; Tom Coghlan, Beirut

October 6, 2016 - 12:01am
The Times

A resurgent al-Qaeda is plotting attacks against Britain with renewed vigour as Islamic State comes under pressure, sources said last night.

Sir Michael Fallon, the defence secretary, also told The Times that the terrorist network posed a “very direct threat” to the UK and Europe.

An offshoot in Syria — the Nusra Front, recently renamed the Fateh al-Sham Front — is regarded by the West as the most potent, with senior leaders believed to be on the ground actively working on attack plans. 

[...]

The Daily Mail headline on the same story:

Al-Qaeda is plotting terror attacks in Britain and Europe and is just ‘biding its time’ waiting for ISIS to collapse, reveals Defence Secretary Michael Fallon

• Fallon said Al-Qaeda posed a ‘very direct threat’ to the UK and Europe 
• New charge of terror is believed to be led by Osama bin Laden’s son
• He has recently issued audio messages aimed at challenging ISIS' hold

[...]

"So far we see in the actions of primarily Western countries no concern for the humanitarian situation in Syria, but the desire to protect al-Nusra and affiliated forces and militant groups."
-- Russian Foreign Ministry spokeswoman Maria Zakharova, from her press briefing today

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