Hi,
I have this cover:

The cover is postmarked Leeds on 17 August 1975 and was sent to Sweden from the UK with 10½p worth of Machins, all of which are perfins "LC".
I cannot tell who the sender was but the receiver is a Swedish museum.
The cover is marked "T 8½ over FS 10½" which I presume is a postage due marking.
The Swedish label seems to say that the excise(customs duty?) due is 210 ore and needs to be paid in stamps?
The cover is large measuring around 253mm wide by 178mm high. I do not think the contents very very thick though. Probably no more than a few pages of paper I'd guess.
A few questions: The perfin "LC" would that probably refer to Leeds City in this instance?
Second question: can anyone help me with deciphering the postage due marking and how to rate this cover. I have been all over my GB postal rates book and cannot make it work.
Third question: Could anyone help me understand the label the Swedes added?
Thanks and I appreciate the help here.
-Larry