As a new wedding photographer who is just trying to establish his career and finding his way in the competitive niche, your first line of defense would have to be your wedding photography gear. This is not something that you should take lightly. You need to go out of your way to make sure that at some point or so, you are really able to think things through and prepare everything that you could ever possibly need before you head out there and book your first big client. The wedding photography gear is also the most expensive starter kit for your career so you need to make sure that you are able to pool enough funds for it as much as possible. You will need to deal with nothing but the basics at the start. Do not overstretch your budget and stick to nothing but the bare necessities as much as possible. Anything more than that and you end up going broke over things that you do not exactly need in the first place.
Your cameras are your most important tools for photography.
However, when you come to think of it, the fact that you are pursuing a career in London wedding photography already pretty much speaks volumes about how passionate you are about it. You most likely already have your main camera covered. It is just the second camera that you will need to worry about. If you find it hard to afford one just yet, there is no need for you to stress over it at some point or so. You can always talk to your friends and borrow a second camera to serve as your insurance or backup. You can also try to lease it from places that rent them out for a reasonable enough price.
You will also need to invest in a medium zoom lens if you cannot get a telephoto lens and a wide angle lens just yet.
Specialty lenses are extremely important and you cannot really afford them all in one go when you come to think about it. Getting a medium zoom lens means that you are getting something that can more or less do a little of both. It means that you will be able to get the job done without breaking the bank for it.
You will also need an entire army of batteries and memory cards.
You should never run out of memory cards and batteries. A good wedding photographer never does. Bring in more than enough memory space and camera batteries than you think you could ever need. Make sure that your memory cards are wiped clean before you use them to make sure that you do not end up with any malfunctions such as the files getting corrupted, and so on and so forth. Make sure that your batteries are all fully charged as well.
Make sure that you invest in a high quality camera bag as well.
You need something sturdy and padded and something that can protect your individual photography gear. As a wedding photographer, you will be on the move all of the time. You need to make sure that your gear does not break in the process.