How to wire a 13Amp UK plug for a type 2 electric vehicle charging cable?
It looks like there is a plug overheat detection system, with something like a thermistor between the red and white wires.  That will be buried inside the plug somewhere.  If you don't connect the wires, then the charging unit will probably refuse to work.
You also have the problem that a standard 13A plug isn't designed to take 2.5mm² cables.  While it may be possible to jam such a wire into a plug, the cord grip won't work.
Unfortunately, you chopped that wire off rather short.  I suspect the only solution may be to strip the insulation off the stubby little bit of cable, and make a secure join to the end of a new 5-core lead.