In theory all mortgage lenders can help contractors obtain a mortgage. If you are able provide two to three years accounts showing the correct amount of dividends/net profit and salary to support the level of mortgage required, then most lenders out there should be able to progress your borrowing request. Equally so, if you have worked through an umbrella organisation for a reasonable period of time, and have payslips showing the required amount to support your borrowing, then again most lenders will be happy to assist you.
In reality, for the majority of contractors it is not easy for them to provide the necessary documentation to support a mortgage application in the conventional way. What became apparent is that as the contractor community started to grow and develop many conventional lenders recognised that applying criteria from the past would preclude a number of potential borrowers from securing funding. Historically contractors would utilise more obscure lenders rather than High Street lenders for the very reason that they could not meet the criteria imposed upon them by such lenders.
Conventional High Street lenders started to see the contractor market as an area that would provide them with good quality business at a risk that they were prepared to accept.