Summarize:

When you have an idea of your budget, criteria and where you want to start looking, it's time to begin trawling through the online listings. If you are not already based in London, this is the most practical and easy way to see what properties are available where, and at what price. There are many big properties websites that collect postings from numerous Estate Agents and independent landlords, such as RightMove and Zoopla. You can also look through the online listings of specific Estate Agents. Bear in mind that good properties go very quickly in London, and you shouldn't rely solely on websites. Emailing a response to a listing on a website doesn't mean that your email will be read or acted on. If you find somewhere you like the look of, be sure to phone up the agent directly. If you are in the area, local newspapers and magazines can be good sources for rental listings. Keep in mind that the rental market moves so quickly in London, that desirable properties can go quickly. If you see something you are interested in, be sure to contact the vendor quickly to arrange a viewing. A good and time-efficient way to find properties is to visit a local Estate Agents in person. If you know what you want you are looking for, being able to talk to somebody face-to-face can be the best way to discover what is available. Building a relationship with an Estate Agent may also help you receive notifications about new properties that fit your criteria before they are listed online. You should understand that there are usually significant fees for working with an Estate Agent. If you are uncertain, find out the fees before you start booking in any viewings. An alternative to working with an Estate Agent is dealing with a private landlord. This can be a riskier option, but it can also be cheaper and more flexible. Private landlords advertise their rooms on websites such as Gumtree and SpareRoom, and will generally not apply the agency fees that an Estate Agent would. If you are just looking for a room in an existing house share, this can be a good way to go. This option is best for single people, rather than couples. Only rent from a private landlord who you trust and consider reputable, and ensure that you have a proper contract and your deposit is protected. Try to use landlords that are registered with a professional body or accredited organisation. In London there is the London Rental Standards.
Search for online listings. Look in local newspapers and magazines. Visit an Estate Agents. Contact independent landlords.