From the Valid Tracking Rate help page in Amazon Germany:

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VTR exemptions

We don’t calculate VTR for the following seller-fulfilled shipments:

• Domestic shipments below €15: Shipments from an address in Germany to a delivery address in Germany that have an order value below €15 (including shipping, excluding VAT) and are sent with an untracked ‘letter’ ship method.
• Cross-border shipments below €20: Shipments to and from the UK, Spain, France, Italy, Netherlands, Portugal, Poland, Austria, Sweden, or Belgium below €20 (including shipping, excluding VAT) are exempt from VTR if shipped using a ‘letter’ method.
• Digital products such as audio books.
• Self-delivered shipments (that is, shipments that are delivered using your own logistics).

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So I asked seller support (on Amazon Germany) what options were considered letters for UK>Germany shipments, and they told me to use Royal Mail 1st/2nd Class.

Anyone got the answer?

The actual shipping service we are using is DHL Globalmail, which is delivered by Deutsche Post as regular international priority business mail.

I have tried using 'Deutsche Post' as the shipping provider and 'Globalmail' (custom input) and 'Kompaktbrief International' (small international letter) but I am still getting VTR hits of 'no valid tracking' for shipments under €20 ex. VAT inc. shipping.