Yep, that is the new law...
I applied for SA citizenship in March 2016, when the rule was 5 years on PR. In September 2016 they updated the application to say I was only eligible for citizenship in March 2010.
Quite bleak that they never told me this before I applied.
You and many others.
To address the OP; it's not a costly exercise, just incredibly bureaucratic!
The British Embassy in Pretoria will only provide a general letter, more of a statement, that HMG has no objection to its citizens holding dual-citizenship. This seems to be acceptable to DHA.
You will be asked for a Police Clearance from country of origin. You apply for this online. Expensive it is!
But as Naks found out, DHA moved the goalposts around Oct last year and introduced 10 years (not a problem for the OP). But the process became incredibly bureaucratic.
There is a whole new process, which kicks off with having your PR validated before the application is even accepted. Then you will have to take a language proficiency test, they don't say how they will handle native English speakers...
What is truly interesting is that these changes came in with the 2012 Regulations (to the Citizenship Act) only introduced in 2013, but curiously, never applied by DHA offices until Oct 2016!
What is not generally known, is that the Regulations contradict the Act, making the regulations unlawful.
Regulations are subordinate legislation. It means they can never be more restrictive than the enabling legislation which is the Act. If it is, it is
ultra vires the Act and unlawful and should be set aside.
An SA attorney (De Saude) suggests that if the matter were to be taken to court, the regulations would likely be set aside...