@Butterfly44 Great question! Can you find out exactly what your child struggled with? For example, if it's English, is it creative writing? Is it analysing unseen poetry? Is it to do with exam anxiety? Did they run out of time? Do they need to get a particularly high grade to access further education?
Then you can hone your search for tutors who specialise in exactly what your child needs help with.
In terms of tutoring agencies/websites, you are right there is a huge range! To be able to advertise my services on these websites, I had to go through quite rigorous identity and reference checks, including proof of address and proof of qualifications. You can rest assured, people advertising through these agencies are verified.
tutorful.co.uk
outschool.com
firsttutors.com
I'm sure there are other great agencies too, these are just the ones with which I have personal experience.
If you choose to go directly to a tutor and not an agency, you have the right to ask for all the evidence you want! I am always happy to provide a copy of my Enhanced DBS check, CV, references etc.
With regards to online/offline, I really think it depends on the individual. Your child might be super receptive to online tuition, or they might not. Involve them in your search. Arrange a trial lesson with several tutors, and let your child feedback to you what they preferred.
Hope this helps, if you have more questions go for it!