I don't think any Company is allowed to sack someone for genuine illness time off. If your Company has a 'clear absence policy' then somewhere in that Document should state the exact reasons for absence that would lead to the employee being dismissed! This is giving mixed messages.
If a person is not pulling their weight due to illness is a different situation but as the Company is aware that you have and will be taking time off, it's up to them to arrange cover for those times, it's not like it's an emergency. CAB would be my point of call or my Solicitor as I don't believe in Unions because they have so much power, however ………