Before you reapply
What must change
in the file
Not everything. The ground the officer ticked, answered directly, and the rest of the file tidied so it raises no new question.
Funds. Statements that show the trip is affordable without emptying the account, a regular income that matches the spending, and a written explanation for any large deposit. Where the picture needs to change, that takes months of statements, not a new printout.
Ties to the UK. An employer letter with leave approved for the dates, your tenancy or mortgage, family here, and the status you hold, presented so the officer sees a life that you will come back to.
Purpose. Flights, accommodation and a day by day itinerary that agree with each other, for the right country and the right dates, with a cover letter that answers the refusal in one short paragraph: what was doubted, and what is different now.
Insurance. Cover of at least EUR 30,000, valid across the whole Schengen area for every day of the stay, in your name, with the certificate in the file.
When to reapply. As soon as the weakness is fixed, and not before. An itinerary that did not match can be corrected this week. Finances that did not convince need time to show a different picture. Reapplying the day after a refusal with the same file is the one thing we will always advise against.