Pre-Employment Background Check in the UK

Any one knows how the pre-employment background check is usually done in the UK? I'm in professional service.
Just wondering how far back would a company ask to verify in terms of employment history.

They usually want your previous three employers.

They usually want your previous three employers.

Thanks!
Would they care about part-time jobs, on campus jobs and research projects at school? I graduated in 2014 and worked for only one employer from 2014 to 2017. I honestly don't even remember the exact dates for part-time jobs I had during uni. 

Thanks!
Would they care about part-time jobs, on campus jobs and research projects at school? I graduated in 2014 and worked for only one employer from 2014 to 2017. I honestly don't even remember the exact dates for part-time jobs I had during uni.

I would think they only want the ones relevant to your career (so just the one).  But you can always ask.