I'm sure there's tax benefits but wouldn't it also reduce fees and increase flexibility due to founding company's control of bank?

Yes, Virgin Money does a range of retail financial products. Their residential and buy to let mortgage offerings are based on their 2012 purchase of Northern Rock from the British Gov't. Northern Rock collapsed during the 2008 financial crisis due to the poor quality of its loan book and loss of access to wholesale money markets. Borrow short and lend long to sub prime borrowers turned out to be a bad strategy! I've just come off a year long contract working on the mortgage origination software Virgin Money inherited from Northern Rock. If I learned one thing it's this: the UK mortgage industry is technologically stone age!