Hi Rob,
The Air Ministry specification laid down that the Squadron codes, example 'RN', be displayed in front of the roundel on both sides of the fuselage. This means the codes reading correctly on the port side, example RN * F, and in reverse on the starboard side, example F * RN. Reason for this was the 'standard' position of the fuselage roundel, set a specific distance, which was ruled by the aircraft type, size, shape etc.
The codes were normally applied on the Squadron; in most cases, the 'rule' was followed but, as in all things, there were exceptions, and some squadrons applied the codes in the same order on both sides of the fuselage.( there is at least one squadron who got it wrong totally, by applying the individual code letter in front of the roundel on the port side and starboard side!)
Bottom line - check references for a specific aircraft, or squadron practice, before committing paint/decal to model!
Hope this helps,
Cheers,
Terry.