There is always a speed limit in force, but the highway code is not the source of law on them.
For example the Highway Code says:
Rule 124. You MUST NOT exceed the maximum speed limits for the road and for your vehicle (see the speed limits table). A speed limit of 30 mph (48 km/h) generally applies to all roads with street lights (excluding motorways) unless signs show otherwise.
So given Rule 124
- what would be the speed limit on a dual carriage way A road, which had street lights and no other signage?
- what would be the speed limit on an urban B road with street lights set 200m apart and no signage?
- what would be the speed limit on a single track road with a single 20mph speed limit sign and no street lights?