Personally I would not crimp solid conductors.
To produce a correct cold weld involves several critical success
factors; a crimp tool of the right type (single indent for fine
stranded, circumferential impingement for stranded/solid); precise
force applied consistently; correct crimp design for the conductor
type (solid, stranded, fine stranded); crimp maker matched to tool
maker (often forgotten).

Personally I would never solder.
Most people can learn to solder with even a little practice (avoid a
dry joint), and learn to bind two solid conductors together with fine
copper wire so the solder does not take the current & fault current,
and learn to adhesive heatshrink (preferably in the appropriate
colours). However for extending a CU it is a bit messy.

I would always use DIN rail terminal block in a suitable enclosure.
The CU uses such terminals, they are easy to access, inspect,
maintain, alter, extend, reduce. Just bundle appropriate cables at
entry to the enclosure with P-clips sized to hold the cables tight (so
no load can be imposed on terminals), or route cables in trunking,
etc.

You can get DIN rail terminals & enclosure from Ebay, CPC, Farnell,
RS, Rapid, electrical factors, lots of places online. The box may cost
about £8, the DIN terminals and end-blocks will cost probably £15-20
depending on number. If in doubt, post the circuits and it only takes
someone a few minutes to suggest the part numbers required.

A novice might screw-up crimping or soldering, and right by a CU it is
a cascading fire, whereas the probability of copying the photo with
screw terminals is both safer and easier - it allows adjustment in
future. Soldering & crimping does not (you end up running short of
cable with changes, something a DIN terminal will prevent).