British film with musical numbers?

Okay, this is another film I have been looking for. I have vague memories of one particular scene, and almost no memory of the rest of the film.

I think the film was maybe a "teenagers at the beach" film. 50s or early 60s, maybe? Maybe it had some pop star and so kept having musical numbers? And I am pretty sure, but not completely, that it was British.

Okay, the scene I remember. At some point the cast go to a cinema. And they watch a movie about the French Foreign Legion. And then a musical number starts up, a pop song. So the cast of the film start interacting with the characters in the film-within-the-film. Singing and dancing. The French Foreign Legion characters come out of the movie and dance in the cinema. And maybe the teenagers go into the film and sing and dance? At some point one of the teenagers, maybe the big pop star, crosses his guitar with the rifle of one of the French Foreign Legion guys.

Nobody has answered this at all!!!

I will go over what I am (almost) 100% sure about, and what I am not certain of.

100%:

Some people are watching a movie, in a cinema.
In the movie-within-the-movie there are soldiers, in uniforms, with rifles and kepis (the type of hat worn by the French Foreign legion)
The soldiers in the movie-within-the-movie are stationed somewhere sandy.
A pop or rock number starts up in the "outer" movie.
The characters in the "outer" movie start singing and dancing.
The characters in the "inner" movie join in.
There is dancing, and the line between the "outer" and "inner" movies gets blurred, people cross over from one to the other.
A young man in the "outer" movie has a guitar, and he crosses this with the rifle of one of the soldiers from the "inner" movie.

Somewhat certain:

There were other musical numbers in the movie.
It was a movie, and not a TV show.
The characters in the "inner" movie were the French Foreign Legion.
The movie about the French soldiers is nor featured or referred to at all in the rest of the "outer" movie

Vague:--things that I have almost no certainty about.
(These are all about the "outer" movie)

The characters in the movie were teenagers, or otherwise considered the "younger generation".
The movie had other pop/rock songs.
The movie mostly took place on a beach.
The movie was in black and white (I may have just seen it on a black and white TV)
The movie was British.

In other words, I am really just vaguely remembering one musical number from a longer movie that I have almost no memory of at all.

Does anybody recognize this now?

I vaguely remember seeing this on TV in the late 1960s. It might be Every Day's a Holiday (1965; US title Seaside Swingers), but I cannot find confirmation that it has the scene inside the cinema that you describe.

At the moment I don't want to pay to watch Seaside Swingers/Every Day's a Holiday, but I am not finding any discussion of that film that references the scene I remember. The movie may not have been British. Remember that there is a whole genre of movies called "Beach Party". And I am not absolutely certain that the movie I am looking for was in that genre!

I rather suspect that a lot of people back in 1965 may have felt regret over having done so.

In any case, I am pretty confident that it is not your movie, based on Peter Viney's description (https://peterviney.com/film-theatre-...ays-a-holiday/).

However, he also reviews a number of similar movies (which, to a certain degree, I think I have conflated). Of these, Wonderful Life (1964; US title Swingers Paradise) looks like it's your movie, as it does indeed feature French Foreign Legion film sequences within the overall movie, plus singing and a whole lot more (https://peterviney.com/film-the-60s-...onderful-life/). If you really do wish to see this movie again, just look at the pictures Mr Viney has posted, as there a lot of spoilers in his review. But be aware that he points out, "It was a major surprise to me, in that I think in some ways it’s very good indeed... Where its reputation fails is the original music, which for a musical is a serious problem."

I am now almost certain that it HAS to be Wonderful Life/Swingers Paradise 1964. (Although there are many other movies with the words "Wonderful Life" in the title (Including "It's a Wonderful Life"), and "Swingers Paradise" was also the title for a series of porn movies)
The film is about the making of a movie about the Foreign Legion. But the "kids" want to turn it into a musical. There are lots of scenes in projection rooms (technically not cinemas), where the characters watch the scenes they have been shooting. The scene I am remembering may be a muddle of several scenes from the movie. I might actually pay to watch this one, I am starting a new job next week.