Compare each service
The next step will be to find out what each of your shortlisted office design firms offer as part of their package. Whilst one company may claim that they can perform the job you need at a cheaper price, this does not necessarily mean that they are able to offer the same range of services.
Does the company you're looking at oversee the entirety of the project, or will you have to manage certain parts of it yourself? What kind of graphics do they use to create designs? Do they offer furniture supply or electrical and data installations? Make a detailed list of all of the services that each company offers and once you know them, you will be able to narrow down your options to find the companies that suit you best.
With any office refurbishment in London project being a major investment, it's crucial that the financial implications of working with each company is considered. Fundamentally, you will need to know that the company has the means to complete the project within your agreed budget. Find out about the financial health of the company; ask them to provide you with evidence that they are in a strong position to complete the project. Do they borrow money? Do they have the right insurance? Do they negotiate with suppliers for materials themselves, or will you have to do it? Any reliable office design company would only be too happy to provide such evidence, easing your concerns.