@securespark has given a good answer, in essence a spur and a radial are the same, but we have English to contend with, if something is final as with ring final, you can't have a final final in English same as saying the very end, it is simply the end.
So a fused spur is really a new circuit, and a radial from the ring final, but English will not allow us to have a new circuit from a final circuit so we invented new words and phrases to label them.
So we have:-
Ring final - where sockets are supplied by two cables arranged in a loop. In the main using 2.5 mm² cable and a 32 amp MCB/RCBO. There may be old installations using 30A fuses, but to extend such a ring final one would need RCD protection so not straight forward. Maxium lenght is around 106 meters.
Radial final - Can be 1.5 mm², 2.5 mm² or 4 mm² with a 16 amp, 20 amp or 32 amp MCB/RCBO with 32 amp one needs to be careful to continue to use 4 mm² (or 6 mm²) with a 20 amp radial using 2.5 mm² maxium lenght is around 32 meters, which is why not popular.
A spur - uses a smaller total cross sectional area so typically 1.5 mm² or 2.5 mm² when comming from a ring final a single 2.5 mm² means half the cross sectional area as only one cable used, the spur is sub divided into a fused spur (really a radial) and an unfused spur, with the latter you are only allowed a single device, this means if people have followed the rules, you can identify an unfused spur as only one cable goes to it. However, this relies on those before you following the rules, with a fused spur, you can have as many sockets as you want.
What I have done when I have found a doner socket supplied with an unfused spur, is to add a fuse.
Using the LAP grid switch system, a socket, fuse, and switch will fit in a double socket box, shown here where I wanted to extend an unfused spur circuit.
The regulations allow circuits installed under a previous edition to remain as they are, but if you add new, then it needs to comply with the current edition, so if there is no RCD protection you need to add it. With surface cables easy enough you just use a RCD socket, but with buried cables the cables also need protection, so using a RCD FCU (fused connection unit) is likely the easiest option.
Of course we should complete a minor works certificate,
link here filling in the certificate serves as a checklist, it does say inquiry is a valid method to get readings, so most of the certificate you can copy the readings given in the installation certificate,
is important, as is
and if on a ring final also
It does not really matter if you can't measure less than 1 Ω what you are doing is confirming it is a ring.
For the electrician it is easy, we use our expensive meter,
and write down the readings, shown here testing the RCD, so we can take a loop impedance reading before starting, (this also tells us how long the cable is so not exceeding 106 meters) and note reading, test the continuity while fitting the socket, at the socket, and then loop reading again when complete, and this will highlight any errors, like a wire popping out of the terminal, the DIY guy does not have this option, so you have to be very careful to ensure all wires are tight.
But even if these
testers don't show if the loop impedance is low enough, as will pass at 1.9 Ω and the pass mark for a ring final is 1.38 Ω, they will pick up most major faults.
