Your employer may dismiss you if they have a legally fair reason. Reasons for a fair dismissal include being guilty of misconduct or that you aren't capable of doing your job.
Your employer is likely to investigate your sickness absence differently depending on whether you have taken short and intermittent spells of absence or if you have had one long period of absence.
A policy may set out a level of attendance that you are expected to adhere to and your absences will be monitored accordingly. There may also be a procedure you should follow when you take sickness absence. If your attendance is being monitored and there is no improvement an employer may invoke their disciplinary procedure, which could potentially result in dismissal. An employer must follow a fair and reasonable disciplinary procedure. If they do not, you may be able to claim for unfair dismissal
If you are trying to argue that your dismissal following sickness absence is unfair and you have not followed your employer’s policy, your case will be weakened.
If you go on long term sick leave, the employer will have to show that it has investigated your health condition and your ability to return to work. In most cases the employer will obtain medical evidence from an occupational health professional. An assessment will be made about when you will be fit to return to work and whether any reasonable adjustments can be made to assist your return.