Some employers have their own sick pay schemes. If your employer has such a scheme (which must pay as much, or more, than the SSP explained below) you need to read your contract. Every scheme is different.
If your employer doesn't have their own scheme, you're only entitled to SSP ('Statutory Sick Pay'), which applies to everyone earning at least £95 per week.
With SSP you get nothing at all for the first 3 days of sickness. Thereafter you get £79.15 per week. If you're only away from work for a few days (after the first three), your employer will calculate a daily rate, based upon how many days you work each week. So, if you work 5 days per week, you'll get £15.83 per day.
Your employer cannot insist that you provide proof of your illness during the first seven days. However they may insist that you 'self-certify' your sickness using the employer's own form or the 'SC2' form which is available from doctors' surgeries (without seeing a doctor or nurse). On the 8th day (or thereafter) your employer has the right to insist that you provide evidence of your inability to work, usually in the form of a certificate from your doctor.
See here:
http://www.direct.gov...llorinjured/DG_175850
and here:
http://www.direct.gov...orinjured/DG_10018786
Chris