Backbenchers form the majority of MPs on both sides in the House of Commons, but the extent of their effectiveness is questionable given the power of the executive
One way in which they are not effective is that they are mainly controlled and curtailed by the whips system, meaning despite revolts on 37% of divisions between 2010 and the present, the government has only been defeated 7 times in the Commons. The rebellions rarely exceed a dozen of the most radical Tory MPS, and the governments working majority of over 70 means they are rarely effective at forming a resistance to the power of the executive. However the few defeats there have been are often significant- for example the 2013 Syrian civil war motion was defeated by 30 Tory rebel MPS and this in turn stopped the US going to war- seriously affecting global geopolitics in the Middle East.
Backbenchers introducing private members bills are also constrained by lack of time. If the executive does not grant a private members bill adequate time for debate it will rarely get through the many stages of the legislative process. For example in 2012 Douglas Carswell, who was a Conservative MP at the time, proposed a private members bill to repeal the 1972 European communities act. It did not pass the first reading. The executive is solely in control over private members bill and very few will ever pass unless they support it at which point it ceases to be a private members bill in all but official title. According to the Guardian between 2009-2010 of the 77 private members bills tabled, only 7 received Royal Assent. However private members bills are often effective at making a point, the previously mentioned example proving Douglas Carswells euroscepticism.
Select Committees are a relevantly recent innovation of the House of Commons, and they allow backbenchers to hold inquires far more intelligent and probing than the spectacle debates in the Commons debate chamber. Often these committees are very effective, for example an inquiry into the preparedness of the government for the 2010 Icelandic volcano by the Science Select committee likely influenced the executive to be more prepared in the future. However findings of select committees are rarely as publicly known as debate chamber sound bites, so few know of the work they do and thus politicians are at liberty to ignore them as they see fit.
Finally a post 2010 reform of the House of Commons introduced the Backbench Business Committee, which is a forum for backbenchers to debate topical issues of the day. A prominent example was when on November 20th 2014 Conservative MP Steve Baker conducted the first debate in the Commons on money creation for over 150 years, a debate supported by prominent backbench MPS Michael Meecher (Labour), Douglas Carswell (UKIP) and Caroline Lucas (Green). The debate was widely watched on Youtube and educated many on the flaws of current monetary policy in a way not hindered by party whips or political correctness. However again due to limited time, and many debate motions wishing to be discussed, the Backbench Business Committee is limited in their ability to serve the interests of all backbench MPs, let alone the large number of e-petitions
In conclusion, while backbenchers are having more of an influence in recent years, the executive still has a strong stranglehold over them and their activities, and thus they are really limited in their effectiveness in shaping world events.