I seem to remember seeing a science fiction movie in the late 1970s or 1980s which seemed very cheap.
As I remember, the eagle spaceships from Space: 1999 were used as alien spaceships instead of the flying saucers you would expect. I think the plot had something about aliens coming to Earth up to no good.
I'm going to take a leap and suggest you are looking for The Sheriff and the Satellite Kid from 1979. It was a low budget film about a humanoid alien played by Cary Guffey (Yes, Barry from Close Encounters of the Third Kind (1977), talk about being type cast.) stranded on Earth. It does use an effects shot from Space: 1999.
The film is available on YouTube here:
The effect shot from Space: 1999 appears at 1 hour 24 minutes, almost at the end of the film. If you look closely you can see two hawks flanking the large warship seen in the episode “War Games.”
Also worth noting that an episode of Wonder Woman used footage from Space: 1999. Wonder Woman also had episodes about UFO's so it's possible you are conflating them together. The wonder woman episode titled “Time Bomb” used effects shots from several Space: 1999 episodes. The episode with UFO's is titled “The Starships are Coming” but I don't believe this used footage from Space: 1999. It's bang in the time range though, broadcast in 1980 so easy to get it confused.
A very detailed list of where Space: 1999 footage appears in other programs and films can be found here.
Another possibility is the movie The Day After Tomorrow, 1975, starring Brian Blessed and Nick Tate (both were in Space: 1999) and was produced by Gerry Anderson.
Many of the ships were reused from Space: 1999, with slight modifications (though, to be honest, I don't particularly remember an Eagle per se), and the special effects style was pure Space: 1999. Many of the effects people from Space: 1999 worked on The Day After Tomorrow.
To quote the The Day After Tomorrow Wikipedia page:
“The visual style of Space: 1999 provided inspiration to both effects artist Martin Bower, who built the scale models, and production designer Reg Hill, who recycled sets from Space: 1999 to create the Altares interiors.”
…
“They were built from parts of the Ultra Probe set originally made for the Space: 1999 episode "Dragon's Domain", along with design elements from other episodes. Other sets and props were also recycled from the earlier series.”
After searching the list of Space:1999 appearances in various movies and tv shows linked to by skyjack's answer https://catacombs.space1999.net/main/crguide/vclip1.html I think that the movie I am thinking of is probably God Told Me to (1976).
