Wires come in lots of diameters.
But for most basic electric strimmers 1.5mm or 1.6mm diameter is perfect.
Pop off cap.
Cut yourself two lengths of say 3m.
Push ends of both into the little securing holes or clips in cartridge.
Wind on both lengths (I do both wires at same time) in direction show by arrows.
Leave 15cm free.
Push these lengths though the two holes as as you push cartridge back in place. ]
Sorry to say you don't / cannot tell if a pre-loaded one will fit if you do not have name of the strimmer.
Might be time to buy new strimmer.
Not used but I see this says you can re-string it and on sale: https://www.argos.co.uk/product/8863191
Note that there is a bit of a knack to it.
I do not wind tightly. But I do wind both in at the same time (both between a thumb and finger).
As said poundland do sell it, but I never tried it.
I use the 1.6mm from ebay spending about £3.50.
For yours, you wind it all on and then poke it thorough the holes of the belt as you replace the belt (swearing as it suddenly unwinds).
Then you poke it through the two holes in the carrier as you insert into that.
Suggest you use shorter lengths of wire first time you try (say 2 x 1.5m).