While all prosecutors must apply the Full Code Test (see below) when deciding whether to prosecute an alleged offence, different prosecuting authorities may have different motivations and consider different factors when deciding the issue.
Taking the example of the SFO, the Director of the SFO may institute and have conduct of any criminal proceedings which appear to him to relate to serious or complex fraud/bribery and must consider the Code for Crown Prosecutors (the Code) when deciding whether to bring charges. A copy of the Code can be found here.