Parliamentary supremacy is historical rather than legal in source. No Act of Parliament could actually confer on itself the power to make law, as that would be to act on a power that it did not hold.
The common law is the source. It is the courts and the judiciary who have formulated the principle that the United Kingdom's Parliament has unlimited legislative supremacy.
The judiciary obeys the will of Parliament because it is the ultimate political fact upon which the whole system of law making rests.
The judiciary did not invent parliamentary supremacy. Parliamentary supremacy is sourced in a consensus between the branches of government. The content of the principle is determined by consensus, and cannot rest on one branch's interpretation, unless all of the branches agree to it.