(UK) People, especially couples, who argue in front of others or involve others in their personal problems and crises, are said to be washing their dirty laundry in public; making public things that are best left private. (In American English, 'don't air your dirty laundry in public' is used.)
(UK) If something is as dull as ditchwater, it is incredibly boring. A ditch is a long narrow hole or trench dug to contain water, which is normally a dark, dirty colour and stagnant (when water turns a funny colour and starts to smell bad). (In American English,'things are 'dull as dishwater'.)
If something ends in smoke, it produces no concrete or positive result. This expression refers to the boasting by a person, of having put in a lot of efforts by him, for a particular cause or to attain a result which is very difficult to be done by any person. (This mainly refers to an investigation of a crime or solving a serious offence or a mystery). But at the end, when the desired result is not obtained, his claims are found to be false and not worth mentioning. So, he looses his credibility.