Your rights to be paid when off sick depend on what is in your contract of employment. Contracts usually have a clear written term stating what you are paid if you are off sick. Often the contract provides 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 is silent about sick pay the courts might, in some circumstances, imply a term giving you a contractual right to sick pay. In other words, based on custom and practice - that is, because sick pay has always been paid at a particular level, or for a particular length of time, in the past - there may be an implied contractual right to be paid sick pay on this basis even though that entitlement has not been written down.
Especially in non-unionised workplaces, it is not unusual for the contract of employment to state, for example, that contractual sick pay is paid "in the discretion of the employer". Even if the contract contains wording like this, your employer must still pay the statutory minimum sick pay (known as Statutory Sick Pay). And your employer must not exercise any discretion irrationally or arbitrarily, or engage in discrimination, harassment or victimisation.
And your employer must not breach the fundamental duty of trust and confidence owed to all employees.
For all these reasons, most large employers, especially where unions are recognised, prefer to negotiate clear rules that state when sick pay is and isn’t going to be paid.
If you have no contractual right to sick pay, your employer must pay sick pay under a scheme no less generous than 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.