OK - here goes -
What is totally unclear from your OP is what the hell it is that you are selling. IF you are selling perfectly legitimate and original products made by this manufacturer, then you can tell them to go to hell. They are trying it on!
(It may be that they have exclusive main dealers for every area and they seek to protect these areas and your selling of their stuff has come to the attention of their local UK dealer and now they want to put the fear of God into you. As I do not know what it is that you are selling, this remains just a guess!)
What they are after is counterfeit goods and if that is what you are selling, you may (if they go the whole hog) be for the high jump. That is a criminal offence! German companies are extremely litigious - they take people to court just for fun and/or to prove a point!
Without having seen this letter (obviously!) I cannot give you a definitive answer, but it sounds like an initial out-of-court last reminder. If you do not respond to an out-of-court reminder (similar to the English 'Letter Before Action') the potential plaintiff can apply to the responsible court for an official reminder pursuant to paragraph 692 of the German civil code. The court does not (at this stage) examine the case, but merely sends the reminder.
The German courts ONLY send via the Bundespost (federal post office) and the franking stamp and registered delivery have legal status, in that you would be required to act within a certain time after the date. This would be a so-called 'Yellow Letter' because it once was printed on yellow paper. It will be sent to the official address of your company.
If you receive such a letter, you should object immediately if the claim is unfounded, because your opposition forces the court to examine whether the claim exists at all.
Most claims fall at this hurdle because (if the defendant objects to the claim) the plaintiff has to pay all initial court costs in advance.
If you reply to the letter, objecting to the claim, the plaintiff has to stump up court costs to pursue the matter. If you do nothing, the next thing will be a judgement by default.
You can nip this nonsense in the bud by simply replying that you object to the claim, as it is speculative, vexatious and totally without substance or merit and any injury to their trademark (or whatever it is that they are claiming damages for) should have been filed with the English courts as it falls outside of the jurisdiction of the German legal system.
You must point out that you have not traded in Germany at any time and the plaintiff has suffered no genuine loss i.e. the plaintiff has to go to the defendant and cannot just file a case in his home town.
If the plaintiff decides to pursue the claim, here's an outline of what the plaintiff must do to gain title in the UK (this is for the benefit of those of you playing 'The Home Game' and for the Beagles amongst us who like to count the numbers of angels on the head of a legal pin!)
Firstly, they must gain title in their local jurisdiction, i.e. win the case and thereby obtain an enforceable court order.
English law implemented EU Directive 74 (enforcement of judgements in different jurisdictions) on 10 January 2015. In general, the changes have simplified the enforcement procedure and the procedures are less time-consuming and cost-intensive. The reality, however, is that it continues to take a long time for the judgements to be registered and for a corresponding instrument to be issued.
Unlike judgements issued before January 2015, where various documents have to be obtained, the creditor must now 'only' obtain a certificate from the court of origin in the standard form set out in Annex 1 to the Brussels Directive, which proves that judgement is final.
Information about the judgement as well as details of interest and costs must also be included. The plaintiff must submit to the executing authority for enforcement a certified copy of the judgement and the deed of the court of origin. The said authority will require a certified translation of the instrument or judgement, completed by a registered court translator. Nearly always the certified translation of the entire judgement is required. In complex contentious litigation, this can easily be more than 30 or 40 pages of text, which means significant translation costs.
The creditor must submit the instrument and the judgement to the person liable for enforcement, together with a translation before the judgement can be enforced. The enforcement order under the recast Brussels Directive is assigned to the Queen's Bench Division.
After registration, the executing creditor is entitled to enforce the German judgement like an English judgement and all the procedural steps of the English legal system can be used.
All the above only applies if the case falls legitimately under German law. The German legal system requires the plaintiff to go to the courts of the defendant and cannot use his/her local courts unless it has been agreed by both parties that the area of jurisdiction (Gerichtsstand) is elsewhere.
In your case, I strongly suspect that the 'Gerichtsstand' may easily be England or Scotland and not the local German courts. N.B. Germany has 16 federal states and each one is a separate jurisdiction - similar to the US.