Joining 2.5mm T & E

Fit a 13A socket at the place where the cables have to be joined...

Or a junction box.

If the join will be inaccessible, then it must be crimped, soldered, or brazed. Crimping is the one normally used (most reliable in less skilled hands, providing correct crimps and ratchet crimper used).

Hire a licensed professional.

Sorry. From a purely informational point, this wasn't terribly useful. But the correct answer will depend mainly on the codes in your jurisdiction, which you did not specify. Once specified, someone familiar with the local codes can answer, or judge whether providing such an answer might lead to a bigger can of worms than is worthwhile, liability-wise.

Erm, there are no such animals in many countries, including the OPs, judging from the wording of the question and the time zone of the post.

Lets see: 'Ring bus' and a time zone offset 0h from GMT probably puts him in Great Britain or somewhere in Western Europe. Definitely the land of permits, licenses and other sundry rules and regulations.

I don't see any reason why you could not use these type of connectors. As long as the cables were glanded into a suitable enclosure.

Anusol Boy.