Sven, should be aware that the vast majority of the posters here are US bankers. We are (At least I am) unfaniliar with any tax related to funds transfers to or from the UK of the kind you mention.
My guess is that rather than a tax, the buyer's UK bank is charging a fee for the service of sending the wire transfer and converting the buyer's pounds to euros. Most financial insitutions are going to charge your buyers for this service, so I don't see how it can be avoided.
If you do business with the same customers frequently, You could have your buyers pay you via checque instead of a funds transfer. To ensure that you get paid, you would have the buyers execute a standby letter of credit drawn on their bank. If the checque the use to remit payments is not honoured, then you could invoke the letter of credit to ensure that you were paid.
It should be possible for your clients to issue you a standby letter drawn in euros. This would relieve you of the currency risk of receiving payment in pounds and converting the pounds to euros.
Your bank in Stuttgart should be able to help you with this process. Good luck - I hope this was helpful to you.
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The opinions expressed here are personal and do not represent opinions of my employer.