Identifying Mail

I dont know understand well this mail, it would be great if someone explain it better. Why does it got a stamp from france then other from england and japan at the same time?

During the 1971 mail strike in the UK many private delivery companies took over the deliveries and they printed labels (stamps) for their services. Many exotic locations were advertised such as Japan, Israel, etc but mostly they were ferried or flown to Germany, Belgium, Holland or France where a local stamp was affixed, postmarked, taken back to UK and sold to collectors. The label (stamp) had no validity outside the UK so any ongoing journey needed the correct postage stamps added. Your strike label was one of two issued for Japan... one (blue) in the old currency and the other (red) in the new decimal currency. As the currency changed in mid-February, I would expect your cover has the old currency label so that they could use them up and sell off to collectors.

Is it common or rare? What is it called this kind of postage? How much could it worth?

It could be called "UK Strike Mail 1971". Your cover never saw Japan as it seems to have been ferried to France, postmarked, and returned the same day. The mail worker's strike lasted only weeks and many Strike Mail companies tried to cash in with labels (stamps) and mini-sheets for collectors.
These are quite common and I see them on ebay priced at UK £2 to £6 Buy It Now but in auction they never seem to get past the UK £0.99

Thank You I feel that You've said everything and there's nothing more to say. Very Nice description. Thank You