The CAA's recommendations are in Safety Sense leaflet 7,
Aeroplane Performance (114kB .pdf)
From p.3:
Where data allows adjustment for wind, it is recommended that not more than 50% of the headwind component and not less than 150% of the tailwind component of the reported wind be assumed. In some manuals these factors are already included; check the relevant section.
Can't remember the exam, but the usual rule is to multiply all the relevant factors, including the additional safety factor.