The way to get around customs charges is to get to know what the
customs classifications are, and then use them by actual number
The so-called 100-year rule is not always relevant. But old, or
even new, musical instruments carry different tariff classifications.
For example, "pipe organs" and parts imported into the US are free.
And I assume the UK is similar.
I was able to find all the classifications on line, and that really
helps in figuring things out beforehand and then classifying and
"describing" it on the export papers, even for small items shipped
by post or UPS. Or ask for help from a friendly customs broker.
There are often many ways to describe a particular item.