I used to collect coins so perhaps I can help.
The only rare or valuable coins which one might still find in general circulation are the early (1971) ones, but only if they are in excellent ("mint") condition. Before the 5p and 10p pieces were reduced in size, the old shillings and florins were still legal tender and one would occasionally find very old ones in one's change.
The coin which you describe does not match any variety of bona-fide coin that I am aware of, so it could be a very rare type of mistake which does occasionally happen. It sounds as if the 20p coin has been accidentally stamped with the lion design for the 50p. I would advise you to find a coin dealer (there should be some in yellow pages etc.) and show it to them. There is a real possibility that it might be worth a great deal - although there is also a possibility that it is not worth much. The value of such curiosities depends on how well-known they are, e.g. if there is an accidental issue of hundreds of the same thing, and they might become celebrated items. Even if it not worth much, keep it and it might become more valuable in later years.