You obviously have to find an interior designer in London whom you can afford. However, the most expensive does not necessarily equate to the best job. Just be sure to set your budget and stick to it.
Ideally you want to search for interior designers in London who have similar ideas and outlook as your own. It’s important that you do not clash. If you want a bohemian and eclectic feel to your home, you need to hire an interior designer who understands this and will work with you.