This depends on what is in your contract of employment. Sometimes contracts have an express term providing for full pay for a specified period, followed by reduced pay for a further period, subject to conditions on reporting the sickness absence.
If the contract does not contain any such provision the courts could, in some circumstances, imply a term giving a contractual right to sick pay.
Otherwise your employer is required to pay sick pay under the statutory scheme. This is called Statutory Sick Pay (SSP) and is payable, subject to qualifying conditions under the National Insurance contributions scheme for up to 28 weeks.
For more information, see our Sick Pay section.