The most fundamental rule when acquiring street food anywhere: get it when it’s fresh. Minimize bacterial growth by minimizing the time
between the fryer and your gullet.
How to apply this rule in London or anywhere in the UK:
Go out drinking in a place where there are lots of young people partying. Drink at a destination that is 1) busy and 2) at least six
blocks from your nearest mass transit depot. Get pissed with your friends and head towards the mass transit depot at traditional closing time (12:00 am in London), not at club closing time (about 3:00 am in London). Walk to your destination via the most populace route possible. Along the way, you are nearly guaranteed to encounter multiple “chip shops” or “kebab shops”. They are very likely to be selling doner. Choose the shop with either a) the biggest crowd, b) the most people working behind the counter, or c) the least meat remaining on the doner spit. Give preference to places that sell “chips” over places that sell “french fries”.
In London, it seems customary to eat street food on buses but not on the tube . Never leave garbage on mass transit here. Take it with you and drop it in one of the trash bins on the street.
I personally enjoy a small chicken doner with chips. I ask for everything (all of the “salad”) on the doner with no hot sauce. Vinegar, salt and ketchup are essential on chips.