Extend radial circuit with new socket - diagram
It's a radial as
(a) it does not return in a ring to the consumer unit
(b) it's 20A on 2.5mm cable. A ring would be 32A on 2.5mm cable. A radial circuit at 32A would have to be in 4mm cable. Except for pyro cable which is completely different

On a radial it doesn't matter if you 'spur' off a 'spur' as the circuit protection is sized for a single cable, provided you extend in the same size cable as the rest of the circuit.

The former standard circuit with 20A fuse or MCB protection with 2.5 mm² live and 1.5mm² protective conductors was allowed for a floor area of not more than 50 m². I think under 18th Edition IET regulations the floor area is now advisory. 

You still have to consider disconnection timing and Zs if you are running long cables though. 

Edit to add, following comments

This isn't a ring which needs to be "returned" and continued. On a radial circuit you can spur off any-old-how. There are no restrictions provided you use the same size cable throughout. The cable is fully protected by the supply circuit breaker. 

In fact what you propose would increase the total circuit length by twice the distance which would be significantly less favourable than teeing off a single cable to feed the new socket(s), as well as having two junctions instead of one. 

However, you should be sure whether it's a ring or a radial. If there is only one set of conductors at the MCB it cannot be a ring. 

http://wiki.diyfaq.org.uk/index.php/Testing_a_ring_circuit

On a radial circuit (2.5mm / 20A) you can tee off any-old-how. On a ring circuit (2.5mm / 32A) you can only spur off one single or double socket as an unfused spur.