Personally no, I would not use it at all if going TT. You just complicate the issue and make it harder for those that follow to work out what is going on. At 37 ohms its doing nothing of use in real terms
- it would not even trip a 6A MCB on an earth fault.
Assuming you are on class as we are (SE Essex), then yes that is a pretty safe bet - the clay never really dries out once you are down a couple of feet.
You will probably manage it with a lump hammer without too much difficulty IME. I have done 4' ones with no difficulty using a hammer, and got sub 15 ohm readings on those (and 6 to 7 ohms in many cases).
Most RCBOs are type B (you don't always get the choice), however the choice of response curve is more to do with the nature of the circuits you are supplying and nothing to do with the RCD part of its response.
I routinely use type C devices on lighting circuits where there are incandescent lamps in use, and the type Bs on any other "normal" circuits unless there is a particular reason to go for something different due to the inrush specs of something being powered.