For example, a coalition of Conservatives and Labour, as currently this would command a clear majority and represent a greater majority of the population.
Surely representing more of the population is desirable and there is nothing to stop the major parties from being more closely aligned in their politics than with much smaller parties.
There is nothing in law to stop them, but ... why would they? Sure it would let them get things through parliament very easily, but what would they want to get through parliament. The two main parties in the UK disagree on the vast majority of policy areas - that's why they are different parties. If there was a clear shared agenda, they would likely form a party around that.
It's also worth noting, from a practical point of view forming a coalition with the party seen as your main rival would usually severely damage your credibility with your voters, and would likely make re-election difficult.
There are situations where this makes sense. In times of crisis, countries are sometimes led by a so-called "Government of National Unity" or "National Government" (see here and here for a more UK-centric point) but these are very rare as they require a crisis so great that the political differences between the two main parties are rendered irrelevant (in practical terms this usually means a war).
Another point worth making is that in one area of the UK, this is compulsory! The Northern Ireland executive requires power to be shared between the largest Republican party (which is usually also left wing) and the largest Unionist party (usually also right wing). The executive is led by the First Minister and deputy First Minister who have the same governmental power, resulting in a duumvirate. This was designed to ensure that both Republicans and Unionists felt represented in government. The downsides of such an arrangement are now being keenly felt, as the Sinn Féin and the DUP currently have seemingly irreconcilable differences which has caused the power sharing to collapse and has caused a crisis in Northern Ireland, since there currently is no executive and there cannot be one until the parties come to an agreement.
Technically correct but politically almost inconceivable. This is usually referred to as a "government of national unity", and was present during the crisis from 1931-45.
Doing so without a clear national emergency would result in huge outrage from the party base and likely electoral ostracism at the next election.
Surely representing more of the population is desirable
Almost nobody cares about this in UK politics.
This is known as a grand coalition or specifically as has happened in the UK, National Government.
They tend to occur only at times of national crisis, such as wars. In normal times they are unlikely to occur as the ideological differences are too great to maintain unity.
If a store says that you have to pay 51 for something, are you going to ask "Is there any way I can pay 60?" A ruling coalition requires 51% of the vote. These votes are purchased by making concessions. The larger a party is, the more bargaining power they have for demanding concessions. So, no, it's not the case that "representing more of the population is desirable". Strategically, the optimum situation is to represent the bare minimum required to hold power, as that requires the least concessions. In a country like the US where there's a separately elected head of government with veto power, there's more incentive to work towards a veto-proof super-majority.
