There are factors that can be taking into consideration when choosing a therapist which can take time and that's okay. In doing research start with checking if your therapist is on the BCAP register (which can be done online via https://www.bacp.co.uk/search/Register ). Research what types of therapy are on offer and the experience they have. You wouldn't go to a plastic surgeon for a collapsed lung and with counsellors its the same, each have their individual expertise.
After all necessary research now its time to narrow your chosen therapist down and try a first session which gives you and your new therapist some time to get to know each other and what issues you want to work on or what goals you have in mind. If for any reason you do not get along with your therapist or feel comfortable with the proposed action plan of treatment, you can change therapists as that is your right. The best therapist for you is one you have a good rapport with and feel comfortable with sessions going forward.