This letter was supposed to have been sent from the UK, but not through Royal Mail. It looks like one of that commercial letters that someone sends to the post company for they to print (Deutsche Post. But the French language on the envelope is a bit confusing). It was neatly printed on a boring white DIN A4 paper, and it did not bear any real signature.
But there is a thing I found funny. The letter starts:
Thank you for sending us your letter. We're glad you took the trouble to send it, stamp and all. [...]Well, all I can say is... you didn't take the trouble to stick any stamp!