When dining out in London, what is an appropriate tip percentage to leave?
Tipping culture in London restaurants can vary radically from venue to venue, and is largely dependent upon the type of venue you are visiting. If you are dining somewhere with table service, it is generally considered good practice to tip waiters between 10-15% of the overall bill.

In some instances, you will find this percentage has already been added to your bill, which helps to keep things simple and transparent. However, in other cases the rules may be a little more hazy and dependent upon your own choice and perception of the meal and service you received.

When visiting a fast food restaurant, there is no particular need to tip, but there may be a tip jar for coins. The same rules apply at cheaper restaurants, and at street food vendors.
Is tipping as mandatory in London as it is in the usa?
Britain does not have the same tipping culture as America, mostly this can be attributed to minimum wage laws that require all employees—waiters, bartenders, and bellhops included—be paid the national minimum wage, and their bosses aren’t allowed to count gratuities as part of the total.
It’s still minimum wage, however, so you’ll want to drop a few pounds here and there to show your appreciation.
There are some unsaid social rules about leaving a tip that are worth becoming familiar with in the UK. For example, if you take a taxi (cab) it is normal practice to round up the fare to the nearest pound, but don’t expect as much pressure to provide a tip as you might see in the USA.