To be clear...
... Oracle "systematically denies" etc. etc. to American sales reps (allegedly) because there is no way in hell this sort of nonsense could survive under UK employment law or most likely anywhere else in the EU. In fact, you don't even need to invoke employment law to shoot this down in the UK: Unfair Contract Terms Act 1977 would almost certainly suffice.
(NB: The 1977 Act applies to employment contracts as well as per: Brigden v American Express Banks Ltd [2000] IRLR 94 High Court which was another example of an American company thinking they could vary any term of employment "at will")