If your all talking about the MOT side of it, yes you can have brake lights and not side lights, also its the same with indicators, all it says is
"Stop lamps are not a legal requirement if the vehicle is fitted with neither front nor rear position lamps etc. (see 1.1),"
"Direction indicators and hazard warning devices are not required by regulations on vehicles not fitted with any front or rear position lamps (see1.1)"
if they are fitted, they have to work.
This is a good up to date place to go for all the MOT information
http://www.motuk.co.uk/manual/contents.htm for cars and trikes and here
http://www.motuk.co.uk/manual/mc_contents.htm for bikes