Standardisation is possible for GCSEs and A-Levels because there are higher academic authorities than schools who are able to oversee and moderate the examinations. These are the Exam Boards - many of which have their origins with Universities (e.g. OCR) to whom all schools must defer.
Universities do consult one another about standards by using an external examiner from another university as described above, but this is by agreement between equals not by one having authority over the other.
To make an analogy - if we didn't have the weights and measures act and each pub in the country could decide for themselves what a pint is, with no universal standard, you would find that neighbouring pubs within walking distance of each other would tend to use a roughly equal measure for what would be considered a pint because if there were dramatic differences the locals would drink elsewhere, but you can be 100% certain that as you got into the centres of the more expensive cities, the measure being used for a pint would be very different from what it might be in a rural market town, and probably the isolated pub in the middle of nowhere with no nearby competition would also be able to be different. It is only by placing a single higher authority over all such businesses that the law is able to impose that a pint means a pint and that is 568ml.
It is possible to have a standard for a Maths GCSE because although there is more than one exam board awarding the qualification, there are few enough that they can work together to ensure that the standard is the same across all six.
There are 130 universities in the country and they are all independent, authorised to make their own decisions about what standard to expect of a student to be given a degree. There used to be a category for tertiary education providers who didn't have this level of trust and independence and which had to defer to a more senior academic body to confirm the standard status of their degrees - these were the "polytechnics" and this concept was abolished in 1992, with all the former polytechnics being uplifted to independent university status. When they were polytechnics, each one used to have a "proper" university overseeing their standards and telling them if they were getting it right.
You simply cannot get the representatives even for one single academic subject across 130 institutions to all agree on a single standard without placing some in authority over others.
Placing some in authority over others would destroy the very fabric of the meaning of being a university.
So, it can't be done without destroying the meaning of University education.
You certainly could, if you so chose, create a new kind of qualification, to be taken 3 years after A Levels, which was a standardised national examination overseen by a central exam board. It would not be a Degree though.