If it really is a ring and there's nothing else on it anywhere apart from those sockets all in one place, then it's theoretically possible but the result of doing that (two cables in parallel) would be a very non-standard circuit and most electricians would probably not touch it on that basis even if it could be calculated to be OK.
However, that would be a rather unusual setup - are you sure it is actually a ring? Much more common for a few sockets in a garage on their own breaker would be a radial circuit (just one cable). That could certainly be re-purposed as a charging circuit, but most common is for it to be only a 16A circuit - which would suit a 16A chargepoint, but not a 32A chargepoint that you would really prefer. What is the breaker rating that supplies the existing circuit?
You don't really want the chargepoint sharing RCD with other circuits in the CU, but that can be sorted in various ways - your key concern here is with the cable run under the floorboards.
If lifting the floorboards is really a problem (laminated floor laid on top?) then sometimes a circuitous route works out easier, like going all the way up into the attic and down again. But the best way will depend on the preferences of the electrician doing the job - his skills and experience against the details of the fabric of the house.