You will most definitely want to reserve your London hotel room in advance. In less busy locations, you may be able to take chances. However, when it comes to major tourist destinations, you will not want to risk not being able to find a room. The further in advance, the better, as even reservation space can disappear if you wait too long.
If you will be traveling during the warmer months, you may find it harder to get a good deal on a room unless you make your reservation early. This is the busy season for tourists, as everyone wishes to tour the sites without being wet or cold. There is nothing wrong with visiting during the busy season, but keep the need for accommodations in mind if this is when you will be visiting London.
Your London hotel options will be many if you plan in advance. Planning ahead will also allow you to get the lowest priced rooms. If you need to stick to a budget, it is highly advisable that you find out how much your room is going to cost. This, along with your transportation costs, could very well eat up a large portion of your budget if you do not plan carefully. Remember also that tours and dining will add up. If you choose a lower priced hotel, you will have more money left over for fun.