Whichever mortgage broker you choose, make sure they are 'whole of market' so they have access to every mortgage product available to intermediaries. Also make sure they tell you at the outset whether they charge a fee and if so how much that will be.
Some will charge a fee on completion, others will take a small commission from the lender and rely on commission from selling you other related products such as buildings insurance, life cover etc.
There remain a few brokers that still want to charge up front fees before they do any research at all. As you'd expect avoid these at all costs, as any broker worth their salt would be happy to get paid once they've done the work for you.