Funnily enough I never used to use a broker. Did it all myself for years. Then I applied for a mortgage with Tesco bank (when it existed). What a fucking ball ache that was. Took 6 months to get an offer and several complaints. Basically they didn't believe our outgoings and kept asking me how much I spent on haircuts and food. I seriously lost my shit with them in the end.
I also got 500 quid of compensation off then afterwards. Personally I'd never apply for a mortgage without using a broker now. It all just goes so much smoother when you use one and they know where to place the business dependant on the case, i.e above commercial/self employed/ whether it'll meet a new build definition and you need an increased deposit etc