As some couriers may not cover guitars, or the cost of insurance can be ridiculously high, once you have found a courier, you could have a look at insuring through secursus https://www.secursus.com/en-gb/ They cover international shipments as well as domestic.
I have used them several times when sending within the UK with parcelforce, however I have not had to make any claims so I can't comment on that process.