From the workingfamilies.org,uk factsheet:
Sick leave is unlike other forms of leave in that you can eventually be disciplined or dismissed for taking too much. If you have odd days of sickness here and there and your employer believes you are taking more time off than you need or pretending to be ill, your employer may discipline or dismiss you for misconduct.
If you have a long period of sickness or repeated periods of illness, even if your employer believes your illness to be genuine, your employer can say that you are no longer able to work and dismiss you on the grounds of capability.