The brutality of constructive dismissal is that it is entirely at the employee's risk. They have to resign, and lose their employment, with no guarantee that their constructive dismissal claim will be successful.
The premise on which CD is founded is that the contract of employment is effectively 'torn up', there is a repudiating breach and therefore it is as if the contract no longer exists. Therefore aspects such as notice periods are no longer applicable.
This is why CD is technically so complex and the employee rarely comes away with a full pocket of compensation. Precedent is very weak because very often a settlement agreement is cobbled together by the legal team of the employer and the employee is bound to silence, and it doesn't reach Tribunal.
I agree that discrimination is a different claim, with different timescales associated, ie it is applicable from day 1.
OP, the fact you have gone on sick leave at the crucial time, whereby it will tick over into the 2 years will not have gone unnoticed by your employer. They are trying to sound you out, to know what your intentions are. Did you cite CD when you handed in your resignation? btw if you are intending it to be a discrimination claim, then you need to cite harassment based on a protected characteristic, rather than bullying.