Many people enter this field confused, and obviously focussed on their grades. Some do because of a failure to connect with teachers or tutors at school or university, some because they want the benefit of tuition by people who aren’t delivering highly structured classroom education in an institution.
Ask yourself what you want—someone who supplements your ‘normal’ classes, or someone who complements your learning elsewhere? Some tutors offer ‘add ons’ to clear up areas where you are unsure, or modules that you missed in class. In these cases, it really does matter if the tutor is familiar with your specification, past examination questions, and what your school is looking for. Someone who holds the school certification of ‘PGCE’—an educational certificate which conveys professional status in the English and Welsh state sector—may also be important to you.
If, on the other hand, you want someone to do something that isn’t happening in class—to complement your classes, inspire you, give you some confidence, and focus on those areas which you just don’t get by example and a variety of methods—maybe you should seek someone who would otherwise make a living in the independent sector or online.
Independent schools (less than previously, but still in great number) tend to hire people who are good at their subject, and who hold advanced degrees in it, who focus individually on the student, and who are less interested in formally structured classes overseen by state inspectors. That doesn’t mean that they aren’t professional, but that you are getting access to a different set of skills by working with an independent tutor.
I think that there are some warning signs for you to watch out for at advanced and undergraduate levels too. Be careful of people who do too many different subjects, or those who are essentially making some part-time money (it’s usually clear from their curriculum vitae on websites.) Money is also something of an issue; the best tutors work mostly from their own websites and loose groups of fellow tutors, and are going to be better than those dropped in by an agency. They may cost a little more than others.
Many tutors these days work at very low rates, and can’t possibly make money except with a great volume of students; similarly most, but not all agencies overcharge clients and underpay tutors. This will, inevitably, affect the quality of the education you receive.
Remember, you are in a market which is largely regulated by reputation and results. Some graduates, and even undergraduates, may offer a very good basic service, but it is highly unlikely that you will get a lot of added value that you could not have got for free with a little application and research. Equally, some highly qualified tutors may overteach, and some ‘qualified’ teachers may, despite covid’s online delivery, be highly inexperienced or unsure when teaching online. Most really good tutors will offer a trial period and a degree of flexibility about cancellation if things don’t work out.
On a personal note, please avoid any agency or individual who offers to write papers or essays, or who 'guarantees' higher grades, especially on work that you have to submit yourself. These are flim-flam people, and it's often easy for schools and universities to identify them. Plagiarism and passing off the work of others as your own is wrong. No one with any academic self-respect will be offering such services. You will gain far more from a guide who will encourage in you the confidence to be as good as you can be than from someone who offers a quick solution--and your future nerves will be much better off!
Finally, people don’t learn in one way. There are huge and growing numbers of resources online, from audiobooks, podcasts, and free Youtube tutorials, through to an entire ecosystem of differently calibrated agencies, tutors, and teaching organisations. No one size or combination will fit all. Don’t sign up on the spur of the moment, and always do your research. There is something for everyone, but you have to look for it!