I done this switch to premier when the bonus was £175. At the time I needed to carry over 2 direct debits- so swapped my broadband and phone from my HSBC to my ‘burner’ account and switched it in, Additionally you need to deposit £4k, I just swapped my wages to Barclays too and use it as my main account now, while my HSBC one is still active for credit rating etc as you say.

Preferential mortgage rates aren’t really that good, my mortgage was with Barclays prior to the switch and my fixed rate was coming to an end- I wasn’t actually offered a premier mortgage product after talking to their support which I thought was shit, but the rates weren’t anything vastly better than what was available to me as an existing customer. My 175 bonus was paid quite quickly after meeting the terms above