This is what the highway code has to say if you want to be specific.
keep both hands on the handlebars except when signalling or changing gear
keep both feet on the pedals
never ride more than two abreast, and ride in single file on narrow or busy roads and when riding round bends
not ride close behind another vehicle
not carry anything which will affect your balance or may get tangled up with your wheels or chain
be considerate of other road users, particularly blind and partially sighted pedestrians. Let them know you are there when necessary, for example, by ringing your bell if you have one. It is recommended that a bell be fitted
im not saying im militant about it and try to knock them off, i just dont see the mentality to happily ride along at 20-30 in a 40 for mile after mile ignoring the traffic behind. its just inconsiderate.