On a TT system if you're unable to achieve a TN vaule for your Ra and you are utilising automatic disconnection of supply as the sole protective measure then you must rely on an RCD for fault protection. This can be an RCD of any rating (you are not limited to 30mA) depending on your Ra value and its stability. If however you choose a 30mA RCD to provide that fault protection then it is also providing additional protection because of its 30mA rating. Remember, only a 30mA RCD can be used as additional protection, any other rating (100mA, 300mA etc...) will not provide this.
The decision as to whether or not to provide an up front type-S RCD is one that is completely up to the designer. On a TT system, an up front time delayed RCD will not provide additional protection as they are typically rated at 100mA or more, neither will it provide fault protection for a final circuit as it wil not trip within the required time of 0.2s. A type-S RCD can only be used to provide fault protection to distribution circuits that require a maximum disconnection time of 1s (on a TT system that is) although some people, myself included, like to fit them as 'back up' (a term not defined in BS 7671) protection. In other words, if an RCD fails to operate on one or more final circuits then the type-S will provide disconnection... eventually. Bear in mind however that you are only required to protect against single fault conditions, therefore the installation of an up front time delayed RCD is not always mandatory. In a typical domestic TT installation 99% of the time you will not be required to install a type-S, although like I and others have said, it is highly advised.