In the U.K., depending on where you are, you should be able to get a good therapist for around £40-£60 per session. More than that and you're generally paying for nice offices, staff and infrastructure rather than the therapy itself. If those are important to you then by all means pay a bit more but perfectly good therapists are available for a reasonable rate.
I'd look for qualifications - minimum of a Post Grad diploma, some universities are better than others but that doesn't mean the therapist is good, just that their training was good. Look for membership/registration of a professional body BACP/UKCP/COSCA. They all have slightly different registration and or accreditation requirements but the important but is they mean your therapist works to a code of ethics with an external complaint process. They all have "find a therapist" sections where you can search for someone but if you Google "counselling directory" you'll find a website who check therapists credentials before they join.
After that, it's about checking their areas of interest, experience and expertise, and seeing whether you get on/can relate to each other. Some of that you'll get by speaking to them by phone but meeting and getting to know each other in your first session will help. If they push you to quickly talk about your trauma in detail, think you'll be done in x number of sessions (or say you'll be there forever) or are unclear about how they work I'd give them a wide berth.
