Some rally manufacturer teams have been subdivisions of the actual car company (like Hyundai is and Citroen was). Sometimes an external rally team is hired to do the job, but the car company provides the budget (like Subaru/Prodrive and Ford/M-Sport until 2012). Toyota Gazoo started in 2017 by ordering the operations from Tommi Mäkinen Racing, but last year they bought the operations back to in-house.

Current M-Sport is a more rare case since M-Sport’s financial side is completely independent from Ford and they have their own business models (pay drivers, car renting, car selling, spare parts etc) that other teams don’t share. Yes, they are married with Ford who agrees to homologate their cars and gives some support in form of car development, but no actual budget.

So, could Williams or another racing team achieve what M-Sport does and build a Rally1 car? Yes, but only with co-operation of a car manufacturer. It would have to resemble a street model and be homologated with manu’s agreement.