They are called “Chemical Biological Radiological Nuclear (CBRN) Barriers.” and are very much inappropriately used – http://www.opendemocracy.net/ourkingdom/dan-hancox/britains-policing-kettling-20-and-olympic-state-of-exception

there is this RFI for them – https://www.whatdotheyknow.com/request/cbrn_barriers as a FOI, which gives more information about them (The ones I saw were next to Sports city in manchester).