UK 1971 Strike Post To US Via France

Let me know if I am correct:
Sender paid £1.20 at oxford post, then Oxford post transported it to Calais in France where they paid for the normal postage for the US : FF 1.80. (14P at the time)

If I am correct rates from the normal postage to US from UK in 1971, would have been 7.5P (<10g) or 15P(second step), so how the Oxford post chose their postage rate? and did all strike offices applied the same postage cost?

"Oxford Post and Courier" was a private company, not the government of the United Kingdom's Post Office. There were a number of private companies that were created to try to fill the demand for people in the UK during the strike to be able to send their mail. They accepted mail and transported by private means to a place like France where they purchased French postage and mailed it as regular mail at a French post office. These private companies in the UK also created their own Cinderella labels (they are not stamps) to affix to letters that they accepted and also to sell to collectors who were eager buyers. A great many letters that were handled by these private courier companies were philatelic creations, but there was also real mail. Your letter appears to be ordinary mail rather than philatelic but it is hard to determine as one never knows if the sender or the recipient were stamp collectors and were creating something for their collections.

Thanks Kimo.
But was wondering how the Oxford post made their postage rates, £1.20 seem pretty expensive and did all strike offices applied the same postage cost?

..."did all strike offices applied the same postage cost?"
Hundreds of these private companies existed...for a short time... during the postal strike of 1971. Most were in it for the money and could charge whatever they pleased ...or could get away with. Some did a decent job and even flew mail to France or Holland or any near European country to post on the mail. Many, as I assume your cover is one, took the ferry to France and used the nearest French post office to purchase the appropriate stamps to attach to the mail according to the country it was going.
At the end of the strike hundreds of bags of mail were dumped at the doors of some GB post offices for them to deal with. Having made their money from the sale of the labels to customers but mainly collectors, these strike mail companies soon ended their connection with mail "services".

As scotzm points out, these were private companies, many of them were quite sketchy with little or no incentive to do anything but make a quick profit before they disappeared since everyone knew the mail strike was not going to last very long. They could charge anything they thought a person desperate to mail a letter would pay. If you were there at the time, you could put an advertisement in the local papers saying that you would get mail delivered and set your own price. You could use your computer printer or a local printer to make up some Cinderella labels to attach to the letters people gave you, and then you could stuff some suitcases full of them and take a quick trip to France or the Netherlands or any other country where the fare was the lowest cost to get to and then dump them off at the nearest post office with enough money to pay for that country's official postage stamps and let them do the rest of the delivering for you. Or you could just toss them all in the nearest trash bin and not worry about spending the money to get them to another country and pay the postage there since you would be out of business in a very short while when the UK post office ended its strike. These were purely temporary private businesses created suddenly and going out of business quickly and just as suddenly when the strike ended. They had nothing to do with the UK Post. You could even do a similar thing today from your home in Switzerland. All you would need to do is to get a couple of friends to write a few letters, use your computer printer to create some pretty labels, perhaps call them "Codexluminati Post & Currier '17" and decide how what value to put on them - perhaps 5 Swiss Francs or something equally overpriced, then get in your auto and drive over the border into the nearest other country, find a post office there and put some official stamps of that country on the letters and then drop them in the post office box at that post office to be sent onward.

Thank you all for the info. I will add this cover in my collection "withness on our times", with all the explanation.