There are many types of climbing qualifications from indoor awards to people that are able to guide you up Mt. Everest. I would never recommend booking a course with anyone that does not have a Single Pitch Award (SPA) or Mountain Instructor Award (MIA). The MIA is a well respected award that will allow the instructor to teach any aspect of rock climbing anywhere in the UK. The SPA is also an outdoor climbing award but it does have limitations. Find out how will be teaching you and what qualification they hold.
Look into how long the rock climbing instructor has been working in the industry. Some one that has not been teaching for long might not have the experience you thought you were paying for. On the other hand, someone that has held the climbing award for 30 years might not be up to date with current practices. I would look for someone who is a member of Association of Mountaineering Instructors (AMI) as their members are required to take regular training courses.
The price is also an important factor. There is some truth behind the statement “you get what you pay for”. People who have been teaching rock climbing for a long time and are passionate about it will have invested a lot a time and money. High level climbing qualifications take a lot of time and money and climbing instructors would not go throw the long process of gaining these if they were not able to charge more money to their clients. Have a look at what is included in the price of the course – will the instructor provide you with all the equipment, lunch, transport etc. You should understand what you are paying for.
I would also look at group sizes. If you want to learn a lot from your course you will be able to learn more if there are fewer people on the course. A typical group ratio is one climbing instructor to 12 clients. I would suggest that this size of group is far to large for a quality climbing course. These type of courses will be much cheaper but like I said before “you get what you pay for”.