A good summary of using London Taxis and minicabs can be found here
You can always hail a black cab by catching the cabbie's attention (if you can!). They will then pull in to the kerb side to pick you up, but they tend to be expensive.
Then of course there's always the ubiquitous UBER service. However be careful drivers will often employ 'dynamic pricing'. In other words prices go up in busy times and times of high demand. This can often lead to being quoted one price and being charged another when you have accepted the quote. If you subsequently refuse the ride you may be charged a cancellation charge. Buyer beware!