.....the Highway code.... it is an enabling document in British Law and as such is a guide. If it states that you "must" carry out a particular act and then quotes a law including section then it is a legal requirement to do so.
For instance rule 124
124
You MUST NOT exceed the maximum speed limits for the road and for your vehicle (see the table above). The presence of street lights generally means that there is a 30 mph (48 km/h) speed limit unless otherwise specified.
[Law RTRA sects 81, 86, 89 & sch 6]
The requirement to do so though is from the established law and not the Highway Code
Other sections use the word "should". These are guidance only and have no standing in law or otherwise. The only contribution is that it can be used to establish whether a road user is acting in a way that could be expected of a reasonable and competent driver.
125
The speed limit is the absolute maximum and does not mean it is safe to drive at that speed irrespective of conditions. Driving at speeds too fast for the road and traffic conditions is dangerous. You should always reduce your speed when:
- the road layout or condition presents hazards, such as bends
- sharing the road with pedestrians, cyclists and horse riders, particularly children, and motorcyclists
- weather conditions make it safer to do so
- driving at night as it is more difficult to see other road users
You could not be prosecuted for any of these acts in rule 125 on the authority of the Highway Code