The amount of sick pay you get and how long you get it for will be key terms of your employment contract.
The employment contract is an agreement between you and your employer and in general terms, it can only be changed with your agreement. See: Can my employer change my contract?
Speak to your union if your employer wants to cut your sick pay benefits. These kinds of issue are usually best addressed collectively.
If you are not a union member, browse our Union Finder section to find out about the benefits of union membership.