You see that's where we completely differ, your all too willing to put your faith in RCD's instead of making the effort to have a TT system work as it should!! You may well install an S type up-front, to cover any down stream RCD's, but lets be honest now, ...how many UK electricians, let alone so-called DI's would even think about installing one?? Most of them are convinced that RCD's are as reliable as MCB's so why would they bother!! Again, having so easily obtained a 15 ohm reading, means that your ground conditions are pretty good, (and that the existing rod is probably a twig that hasn't been installed properly) so i'd of put a little more effort into the system. Who knows, that TN value could well have easily been realised!! lol!!
To be totally honest, i don't really know why you bother installing a TT system if that's your honest view. If your not bothered about Ra values and the importance of stability, why not just add your RCD's and let any bonding to the installation be your reference earthing. Using your argument the 700 ohm Ra from the existing rod is as good as any, it's under your magic 1667 ohms ...lol!! All in all, it's a sad day, when other international earthing standards can teach the UK a thing or two, especially when in many cases, their standards are actually based or taken on/from past British standards!! Personally I'm happy to install to Bs 7671 regardless of whether or not you agree with it.The reason for upgrading the 700 ohm reading was it was picked up on an EICR by myself as not complying with the recommendation in Bs7671,and coded 3. With a functioning RCD it was perfectly safe and did not represent a dangerous situation,improvement only needed to bring it within the 7671 spec. That is the only purpose of an EICR,picking up things which do not comply with 7671....not the regs according to E54.
On all the projects i've ever been involved on, i can't think of a single one where the specifications didn't call for any earthing system to be higher than 1 ohm. That value covered TX installations, as well as other made earthing systems... and your trying to defend your Reg's and it's 200 ohms!!!! ....Quite amazing!! I think most of the projects you are involved in are much,much larger than the TT systems commonly discussed on here....which are very small scale,such as sheds,single dwellings,garages etc.
As the old saying goes, ...you can lead a horse to water but..... lol!!