It's certainly a grey area, with no specific provision in the law, as bose-x says the only time solo flight is covered in the law is for people who don't have licences.
However, I'd say common sense should prevail, and if the student wanted to bash the circuit, or practice a pre-planned nav, I don't see why they couldn't do this on their own.
Would that solo flight count for FI restriction removal?
I'd say no.