If the Ze of a TN-S is such that the 0.55Ω Zs for a BS1361 fuse (and also maybe even the 0.89Ω Zs required for a BS3036 fuse) (with any cable) cannot be achieved, what would you do - rely on an RCD to achieve the required disconnection times?
An RCD could be used - but to install a new circuit on a TN system knowing it requires an RCD to meet disconnection time would be a very poor design.
Can I ask, what does the disconnection time relate to? I understand Ze refers to the meter incomer, and Zs is at the end of the submain, but 5 seconds seems a long time for a cable to short before it's protective device trips. Does this refer to mechanical damage? (it would be more than 50mm above the plasterboard, and in conduit where it comes down to the CU). Can you protect a 'submain' with a 30ma RCD, then just install MCB's in the submain's CU?
When current flows through cables, they heat up. When a fault occurs, the fault current causes them to heat up significantly. If the supply is not disconnected in a suitable time, they will catch fire and burn your house down.
If 5 seconds is too long, shorter disconnection times can be achieved by altering the design. In many cases, much shorter times would be required anyway.
A single 30mA RCD for the submain with MCBs in the CU does not comply, since all of the CU circuits will be disconnected in the event of a fault. If an RCD had to be used (due to there really being no other option), it most likely would be a time delayed type, and not a 30mA one either.
A smaller value fuse could be used.
Other types of fuse and protective device are available and could be used - provided that discrimination with downstream devices can be ensured.
The measured value of Ze could be used, rather than the assumed one. This would require more regular testing than then usual 10 years to ensure the value of Ze had not changed.
In some unlikely situations, supplementary equipotential bonding could be used.
More than one circuit could be installed, such as extending the original circuits into the extension.
If this is a separately accessed extension, it could have it's own metered supply installed by the DNO or whoever.
The point of this is that once you move away from the small number of 'standard' circuits as typically used in domestic properties, there are many things which must be considered when designing circuits. It is not appropriate to shove cables in and hope, neither is it of any real use asking random questions, since it is not possible to ask questions about things you are unaware of.