Urgent: I lied on my CV. How are employment dates verified in UK background checks?

I’ve recently been offered a mid-level Business Development / Deal Origination role at a global professional services firm. It’s not a director-level or highly regulated position.

When applying, I inaccurately extended the dates of two previous jobs:

• One genuine role was extended by around 6 months
• Another genuine role lasted around 3 months, but I listed it as around 5 months after I was dismissed as i wanted to bridge the career gap slightly as my new role came a few months after.

The jobs themselves were genuine. The issue is the dates.

Please don't focus on telling me I shouldn't have lied. I know that, and I won't be doing this again now that I've built up solid experience in my current role. At the time, I was under a lot of pressure from the job market and felt I had to do what I could to get through the door.

I’m completely fine with education/qualification checks, identity/right-to-work checks and criminal-record checks if they’re required. I’m also fine providing 2 professional references if the process requires them to go through HR/employer verification. If the firm allows professional referees to be contacted directly instead, I can provide 3 references, as one of my former bosses has a very good relationship with me and has covered for me.

My main question is about the UK employment referencing/background-check process for a mid-level role at a professional services firm.

If I get to the background-check stage, would it typically involve:

• Providing 2 to 3 professional referees who are contacted directly?
• Providing my full employment history to a third-party screening company, which then contacts HR/employers to verify the dates?
• Verification of every employer within a particular period, e.g. the last 3 to 5 years?
• Or some combination of the above?

Most importantly, if I put the correct dates on the formal background-check form, rather than repeating the inaccurate CV dates, what normally happens if the screening company notices that they differ from the CV?

I’d particularly appreciate experiences with professional services, consulting, Big Four, advisory or M&A firms in the UK.

I’m looking for practical experiences of how the process actually works rather than judgement about the original mistake.

Would you correct the dates during the formal check and deal with any questions if they arise, or would you continue with the dates originally given on the CV?