Upvote:1
A transit visa - unlike a visitor visa - generally requires you to present the booked/confirmed tickets before applying. Thus you need to book or reserve the tickets first.
For UK, according to official guidelines a Transit Visitor needs:
Evidence that your onward journey from the UK has been booked/confirmed, and that it is either within 24 hours of arrival in the UK for Direct Airside Transit or 48 hours of arrival in the UK for Visitor in Transit such as:
- travel booking confirmation (can be email or copy of tickets)
- travel agent confirmation