Same lender as seller has any advantages in getting a mortgage?

I am a FTb of £305k property with 5% interest, 41 yo, wife 36yo. Been banking with Nationwide for more than 10 years but not main account, two of our credit cards are from virgin which is from 2-3 years. They been increasing our credit limit a lot in the last years, we paid off every month timely.

I also found out the seller remortgage in feb this year with nationwide, there was Bank of Ireland offering 5.24&, but denied to go with Nationwide for 5.42%, over two years there is £350 I will be paying more, this is without any set up fees ( hence increase %).

Is there any real benefit getting mortgage from same lender as seller?