The thing about a spur is it needs heavier cable than a ring - because there's no ring to close. Closing the ring just about halves your cable thickness requirement. Running an extra 12' of cable off a spur might be OK, but it really depends how far the spur has already travelled. 15A cable would be best to prevent overheating.
It's 'legal' but only just.
Safest would be to run a new spur from the nearest point on the ring to the new location, or better still - pull the ring round to include it.
For our transpondian friends, here's a simple diagram of a ring + spurs [if not obvious from the drawing, live, neutral & earth all travel this ring together]…
