UK visit visa
Dummy ticket for a UK visit visa.
The Home Office does not ask for one.
What the Home Office asks for.
Quoted from gov.uk, on the two pages that set the rules.
“you'll leave the UK at the end of your visit”
“you're able to pay for your return or onward journey (or have funding from someone else to pay for the journey)”
“You must have a passport or travel document to enter the UK. It should be valid for the whole of your stay.”
Where a reservation fits.
It is evidence, not a requirement.
Read the second quote again, because it is the one people misread. The rule is that you are able to pay for your return or onward journey. It does not say you must have bought it. Buying a ticket before a decision satisfies a requirement that was never made, and leaves you holding a fare you may not be able to use.
What a reservation does is show your intended dates in a form somebody can read. If your application says you are visiting for two weeks in March, a document naming those flights makes the claim concrete. That is useful. It is not what the decision turns on, and anybody telling you otherwise is selling you something.
If you want the version that can be looked up, that is the verifiable flight reservation. If you only need your route set out clearly, the flight itinerary is the cheaper document.
Nothing here guarantees visa approval, entry, or acceptance by an embassy, airline or border authority. Requirements are set by the authority assessing your application and can change, so check yours with them directly. Read the product disclaimer.
What goes wrong.
Mostly, doing more than was asked.
Buying a ticket to prove you will leave
The rule asks that you are able to pay for the return journey, not that you have already paid for it. A bought ticket proves the same point at far greater risk.
Leaning on the flight document
A caseworker is weighing your circumstances as a whole. A flight reservation is a small supporting piece and it will not carry an application on its own.
Dates that contradict the form
If your application says a two-week visit, the reservation should show a two-week visit. Two different answers to the same question is the problem, not either answer.
Ordering it weeks ahead
A reservation holds for between about 48 hours and about two weeks, and the airline decides which. Order it close to the point where you actually need to show it.