The time/current curves for BS3871 MCB's you're looking for are in 16th edition of the wiring regulations BS7671:1992+A2:1997, BYB, or earlier.
Time/Current Characteristics:
BS3781 5A Type 1 MCB, 0.1 to 5s = 20A
BS3781 5A Type 2 MCB, 0.1 to 5s = 35A
BS3781 6A Type 1 MCB, 0.1 to 5s = 24A
BS3781 6A Type 2 MCB, 0.1 to 5s = 42A
BS3781 10A Type 1 MCB, 0.1 to 5s = 40A
BS3781 10A Type 2 MCB, 0.1 to 5s = 70A
BS3781 15A Type 1 MCB, 0.1 to 5s = 60A
BS3781 15A Type 2 MCB, 0.1 to 5s = 105A
BS3781 16A Type 1 MCB, 0.1 to 5s = 64A
BS3781 16A Type 2 MCB, 0.1 to 5s = 112A
BS3781 20A Type 1 MCB, 0.1 to 5s = 80A
BS3781 20A Type 2 MCB, 0.1 to 5s = 140A
BS3781 25A Type 1 MCB, 0.1 to 5s = 100A
BS3781 25A Type 2 MCB, 0.1 to 5s = 175A
BS3781 30A Type 1 MCB, 0.1 to 5s = 120A
BS3781 30A Type 2 MCB, 0.1 to 5s = 210A
BS3781 32A Type 1 MCB, 0.1 to 5s = 128A
BS3781 32A Type 2 MCB, 0.1 to 5s = 224A
BS3781 40A Type 1 MCB, 0.1 to 5s = 160A
BS3781 40A Type 2 MCB, 0.1 to 5s = 280A
BS3781 50A Type 1 MCB, 0.1 to 5s = 200A
BS3781 50A Type 2 MCB, 0.1 to 5s = 350A
BS3781 63A Type 1 MCB, 0.1 to 5s = 252A
BS3781 63A Type 2 MCB, 0.1 to 5s = 441A
BS3781 80A Type 1 MCB, 0.1 to 5s = 320A
BS3781 80A Type 2 MCB, 0.1 to 5s = 560A
BS3781 100A Type 1 MCB, 0.1 to 5s = 400A
BS3781 100A Type 2 MCB, 0.1 to 5s = 700A
The value of Zs in BS7671:1992+A2:1997, are at a nominal voltage 230V as required in later versions. However, be careful when using the Zs values from old versions of BS7671, the nominal voltage will be 240V. The voltage is relevant because the current required to disconnect in the conventional time divided into the voltage gives the max Zs values for the protective device. A nominal voltage of 240V will therefore result in a proportionally higher Zs max, than now allowed. Therefore, if you have the old 240V nominal, Zs values, divide each of them by 240 then multiply by 230 to get the required 230V nominal values.
The older style MCBs were BS 3871, and were numbered Type 1, 2, 3 and 4.
The time characteristics for the old type are similar but not the same as for the BS EN 60898 versions.
Type 1: 2.7 - 4 In, Type B: 3 - 5 In.
Type 2: 4 - 7 In, Type C: 5 - 10 In.
Type 3: 7 - 10 In.
Type 4: 10 - 50 In, Type D: 10 - 20 In.
During the change over of the standards, manufacturers were allowed to use the new BS EN 60898 designations on the old BS3871 devices, if they were within the limits specified.
For instance a Type 1 is within the specification for Type B.
Types 2 and 3 are within the specification for Type C.
However, Type 4 exceeds the specification for Type D.