The statutory guidance allows for up to 29 days absence in exceptional circumstances, provided that otherwise you are deemed to be meeting the standards.
Remember that the NQT year is based on a holistic assessment of your progress and should not be based on the odd problem or bad observation. Failing to make good progress towards meeting the standards should not come as a shock and late into the year (as stated in the guidance) and if there are issues they should be clearly identified in relation to the standards and help, support and extra training (if deemed necessary) provided.
If you are signed off by your GP then you are deemed ill and being fit and well is your priority now. If the illness takes you over the 29 days it will mean an extra term on induction so that you have a fair opportunity to meet the standards. It would not affect progression in pay etc.