HSE does not have specific guidance for working in temperatures below 13°C.
You will likely be complying with the law if you work in accordance with British/European Standards but you can demonstrate compliance by alternative means.
As a first point of reference you are advised to refer to the British Standards listed below.
The standards listed are not an exhaustive list but provide a framework around which you can develop your risk assessment and start managing the problem. You may need to refer to other standards listed elsewhere, dependant on your operational circumstances.
British standards for assessing cold stress in the workplace