Just do it?
You haven't said what obstacles do you have. Is it a problem with money or finding a proper spot. Maybe you live abroad and don't know how to organise things.There are different issues and lead to different solutions.
You also haven't said if you want just the address or a place that would stand out on the high street so people would notice it and potentially use it.
So I guess you need to ask yourself those questions first and make a plan.
To help you start a bit here I will look at simple scenario: you have limited amount of money, you do most of the work online or at home and you meet your customers only at agreed appointments. So you don't need an office in central London to do actual work, but for the prestige and you wonder how you can afford it with limited budget.
In such a case I would find one of virtual offices solutions. They would give you a great London address for your business and process your post so it would save you some of your time. Some of them offer offices or conference rooms to rent for hours, so you could also meet yoru clients there. All for reasonable cost.