Pest control in the UK is a huge industry this is made up of several very large publicly traded companies, there are also few hundred small to medium family businesses. The remainder and largest majority of pest control companies are local sole traders like myself.
In recent years a large number of call centre companies have started to appear in the Google search results, many of these do not even employ any pest controllers. They give the impression that they are a local company often using a local number, once the booking has been made they call around actual local companies, usually new to the industry sole traders who are struggling to find work of their own. These companies often charge much more than a local company, they take half of the fee and pay the rest to the contractor, so you will paying at least double the going rate.
This is the most important question to ask yourself, can the company actually do what you are employing them to do? You should always ask questions about the process they plan to use, the technicians qualifications, experience and if the work is guaranteed. Having a trade association membership like the BPCA is also a good idea as it shows a commitment to high standards and you have a place to complain to if something goes wrong. Getting recommendations from friends and family is also a good starting point, as are online reviews.
To many this is the one and only consideration. However consider the total cost if you have to pay two companies, when the first one fails to resolve the issue. Generally the larger companies who have shareholders to keep happy will be the most expensive with inexperienced new to the industry sole traders being the cheapest. In most instances the company should be able to give you a fixed price over the phone. If they can't then be careful as the quote will likely be more to do with the cost of the car parked on your driveway than the difficulty of eradicating your pest problem.