It is often a challenge to find good restaurants when you travel. Especially if you are visiting London as a tourist, most restaurants you will be coming across will be close to major attractions and sometimes might not be exactly those London restaurants where the locals go to.
There are several advices we'd like to give:
check Yelp for the latest and greatest restaurants in London - choose by ratings, type of cuisine and location. Yelp ratings can be trusted and are a good indicator of the restaurant level.
some restaurants are walk-in, but others need a reservation. Use OpenTable to make a restaurant reservation in London. It is a free tool, very easy to use, and you don't need to call the restaurant to make a reservation.