The "tanner" (sixpence) was actually a doubled slang term-
"Tanner" was a shortened version of "Tanner and Skin" - Thin
A "Thin" was a(n early?) Victorian nickname for a sixpence.

The 5/- 'dollar' came about because (I believe) the Crown piece (silver before 1947), was approximately the same dimensions as the silver dollar.

And as for quid:
Will Fly states "..."quid pro quo" - Latin for "something for something". ...", quite possible -it's worth remembering that early coins were in silver (weighed in the Troy system, rather than Avoirdupoirs(?)) in which the pount was divided into 12 ounces each of 20 penny-weight (240 penny-weight to the pound - seem familiar?).
If £1 was the ouriginal value of 1 lb of Sterling grade silver* then silver pennies each of one penny-weight seems feasable.

Just a thought.


* An remember just how far back we are going - probably pre-10th Century