Any person who holds an appropriate medical in the UK can be authorised to fly solo by an appropriately qualified instructor.
There is nothing more to it than that.
However, it is impossible to send a student solo in a multi class or in something that requires a type rating.........not cause the law says you can't but because the documents you have to abide by as an authorised FTO or TRTO will prevent it from happening.
IFR can only happen during dual training, a test or when qualified.
Only want to ever fly solo in the UK then simply find yourself an instructor to authorise all your flying and you will never need to bother with any exams or tests. Restricted to licensed aerodromes unless one flies a microlight where there becomes absolutely no restriction though.
When as mentioned before, one wishes to send someone solo to practice some taildragger flying before giving the logbook sign-off then one must remember (for the time being) that the solo has to happen at a licensed aerodrome. The dual can take place at a farm strip provided that the student holds an appropriate licence and class rating and this is only differences training.
Regards,
DFC