"If you don't vote, don't complain. If you don't vote it's nowt [sic] to do with you."This seems to be a ubiquitous sentiment amongst people of various political leanings. It is also something with which I happen to completely disagree.
Firstly, the fact that a person has (correctly) concluded that their vote has a negligible chance of making any difference whatsoever and therefore (rationally) decided that there are better uses of their time* than reading through flyers, manifestos, propaganda and other policy documents does not in any way make their opinions and preferences carry any less weight than someone who has made the alternative choice to vote. Every single person is equally entitled to their opinion and equally entitled to complain, campaign, support or protest in whatever (peaceful) way they see fit, this extends to those who either could not or chose not to vote.
Secondly, refraining from voting can itself be a form of protest. Admittedly, the problem with protesting in this way is it isn't possible from voting turnout figures to distinguish those who didn't vote out of protest from those who were apathetic, lazy or indeed simply had better things to do*.
One could argue that those who don't vote out of principal or in protest actually have more of a right to complain that those of us who do. If you happen to have voted in favour of the candidate/party/policy which wins an election/referendum you have implicitly given your consent to be governed/represented/constrained by said candidate/party/policy and legitimised the use of certain powers and constraints over you and those you care about. Even if you vote against the eventual winner, by participating in the election/referendum process you accept that process as legitimate and therefore implicitly accept the outcome of that process.
Say that you voted for the Libertarian Party at the last General Election. Since the Libertarian Party have no MPs you would feel that your views have little to no representation in Parliament. Chances are high that your representative in the House of Commons is either a Conservative or Labour party member (possibly a Liberal Democrat). Whichever of the major parties they represent, you didn't vote for them. You think that this therefore gives you the right to complain about them, their party and everything they do and stand for. However, you still participated in the process which saw them elected. You just don't like the outcome in this particular instance, in other words you don't like the choices that most other people have made. But, you agreed to participate in the process which resulted in that outcome!
I say free speech to all. This means it doesn't matter whether you vote or not you still have every right to express your opinions in whatever peaceful ways you see fit. Let the non-voters complain if they've got something they think is worth complaining about!
* Some examples of better uses of ones time than voting include: spending time with ones family / friends / loved ones, working to provide for oneself and ones family, spending time at a hobby, doing some DIY/redecorating to improve ones home, enjoying a good meal, enjoying a music concert, etc.