Most of the NHS provision in General Practice is charge free.
There are charges for non NHS work, e.g. medicolegal reports, insurance medicals, travel vaccinations etc. etc. Some of these charges are set, others will be calculated on an hourly base. You will be advised in advance if there are any charges for the work requested. A list of common charges is available at the reception. Usually the person requesting a work is responsible for any relevant fees. Non-payment of fees will at the very least result in refusal to do further private work for you.
The practice operates a complaints procedure in line with that of most other NHS bodies and institutions. A copy of the procedure is available at the reception.