When my strimmer broke, I couldn't find the right replacement part, and similar ones were about half the price of a new strimmer! I have had a few of these break in the past - the spools themselves seem to last.

This particular strimmer is a generic no-brand model, so it is difficult to get cheap spares, but there are many around using labels such as Challenge, Homebase, Powerbase, Tesco, etc.

Actual spare part here:

https://www.amazon.co.uk/Challenge-Homebase-Powerbase-Sovereign-MIGZP250/dp/B008AJOZLI