Ciao people,
As Production Assistant my main job is assisting crew and cast behind the scene and behind camera, but proud to consider myself as exhibitionist, appear in front of the camera as Supporting Artist (SA) or Extra is something I really like to do.
It happened few times during some shift as PA when directors need few SA, which is unusual for me play despite my lack of acting skills, especially with my voice and Italian accent. But despite the fact I am not an actor and I didn’t take such a big part, I really enjoyed. 
I soon realized there are many opportunities as Extra in London, and thanks also to a colleague from Crewsaders, I got a list of some professional crew companies specialized only on Supporting Artists.
So if you are not an actor but you like to appear, make bit money and have the experience of seeing a big budget set, I would suggest you to have a look at some companies I joined which it can be useful apply.
Before you apply, remember to have always prepared some pictures of you on a clear and white background and that you measure (in inches) part of your body like your height, arms etc.
Details to apply are always available in their websites:
  • The Casting Collective (free to apply but a fee of 70£ will be detract at your first salary).

Link: https://www.castingcollective.co.uk/

  • Uni-Versal Extras (few of 30£ per year).

Link: https://www.universalextras.co.uk/

  • We Got POP

Link: https://www.wegotpop.com/pages/uk/

  • Mad Dog 2020 

Link: https://maddog2020casting.com/en/

  • Piece Of Cake Casting

Link: https://www.pieceofcakecasting.com/

  • Services To Film 

Link: https://www.servicestofilm.com/

  • Extra People 

Link: http://extra-people.com/

  • Ray Knight 

Link: https://www.rayknight.co.uk/

  • Guys and Dolls

Link: https://guysanddollscasting.com/

  • Rachael’s People

Link: https://www.rachelspeople.co.uk/

  • Phoenix 

Link: https://www.phoenixcasting.tv/

Before I leave you, I would suggest for who’s interested request a Basic DBS check in order to be more reliable for the companies you apply. The price is 25£, for info and application, please, find the link attached: https://www.gov.uk/request-copy-criminal-record
And last but not least, be clear about your details, what are your skills and if you are comfortable play completely naked or topless. 
Good Luck