I think you meant to say 'what the BSPA consider as having patriality' as this appears to be different to the criteria used by the UK Border agency. If you ask the UKBA what serves to prove UK Patriality they will state that the person needs to hold an Ancestry visa (British grandparent, etc, as quoted somewhere on this thread by Mr Vasey) which apparently only runs for 5 yrs or a Settlement visa (next step on from the Ancestry visa, entitles the holder to the same rights as a UK citizen and after 12 months entitles the holder to apply for a UK passport. This information is very simple to find - took about 5 minutes by talking to their very helpful call centre.
Assume the problem with Cameron is that the BSPA do not recognise or are not aware of the settlement visa as according to the Speedway Star he has one of those (British parent maybe?).
Fully agree with the 'level playing field for everyone' statement but it seems that the BSPA interpretation of Patriality is not 'equal'. If I was cynical I may conclude that this is an attempt to stop a particular rider from riding in the NL.