A mortgage broker, should have access to all mortgages on the market and, as such, he should be able to match the home buyer with the best deal that suits their needs.
That said, some are tied to a few lenders or may push buyers to the lenders that pay them the most commission.
A new home buyer may also need to use a lender that operates the Help to Buy scheme to which may also restrict their choice.
As I said before in another post, you can always check out the best buys yourself so you know what is available and what the charges are.
That is why I have this page!
http://www.brand-newhomes.co.uk/mortgage-current-best-deals.htm