Do you mean taxis or private hire or both?
Do you strictly mean stickers, or any kind of signage required by the rules, or optional stickers that a vehicle may have - for advertising, for example - but which may be controlled by rules?
I think London private hire vehicles require some kind of sticker which acts as the licence plate does in a black taxi.
And both will require stickers to enforce the no-smoking ban.
A black taxi will require a fare plate, but you wouldn't strictly call that a sticker.
Any advertising will be governed by the relevant rules.
Anyway, here are some links I found via Google, but I'm not sure which is the most up to date or whatever.
http://www.tfl.gov.uk/assets/downloads/ ... -taxis.pdfhttps://www.caboperator.co.uk/mogo/sign ... london.pdfhttp://www.tfl.gov.uk/static/corporate/ ... /8798.htmlhttp://www.tfl.gov.uk/assets/downloads/ ... elines.pdf