How to find out what is in pre-production

There are so many projects in the UK that it would be impossible to find out about them all.
You need to do your research.

One of the best resources for this information is found on IMDB pro, its around £150 a year but its so valuable as an actor I cannot recommend it enough.
Its not a casting site its more of a whos who and whats what resource for anyone in the media world.
On IMDB pro you have an option to view all projects currently in pre production, you can filter it down so that its not the entire world but UK specific.
But be careful, I have known certain films that have been in pre production for years and still the movie never gets made.. this is where your research comes in.
Once you find a project that is in pre production, as an actor, I would find out the production company, I would then use IMDB pro to research that company, which casting director they have previously used, research what the producers attached have done before etc
Another invaluable tool of imdb pro is that it gives you company addresses and emails (rare but occasionally happens) this is where you can write or email the production companies direct.
Be warned, if its a big budget studio movie you are 93% likely to not get a response. Ifs its a low budget indie flick then you have a better chance.. so my advice would be to save your mail shot and headshots for those that you stand a chance of being involved in. I would source the attached casting director of the project (for large studio jobs) and contact them for inclusion in the casting process.
You may not get a reply but it wont cause you any harm, however, make sure you send samples of your work if they are not aware of you.

When you find a project you are interested in, make sure you keep the email/letter short and sweet, film production companies in the middle of pre production are busy people.. don't go on about how much work you have done, keep it relevant to the genre of the project you are contacting them about.
For example, if its a martial arts movie and you are highly skilled at Kung Fu, obviously include that in your submission.
Avoid sending files, links are a lot better.. people don't want their email inbox filled up by your fantastic showreel, use links.
Send small files of your headshot.. again for the same reason as above.
Be real, don't big yourself up if you haven't got the credits to support it, even if you have, still don't big yourself up.. just express why you are interested in that project and what caught your attention about it.
Be careful though, as a lot of pre productions in the UK low budget film world never come to fruition. So, defiantly do your research.

If you are looking for US pre production, there are so many its hard to keep track.
Two great resources I use are IMDB PRO (filter to the US) and Backstage. Backstage is a US online service (free) that allows you to see whats in pre production.
In the US a lot more movies in pre production turn around to become a movie than here in the UK. Its a bigger industry over there so more things will go on, even if your not planning on working in the US or getting a VISA, not to worry, its still good to know whos who and where they are at, more often now than ever US casting directors are sourcing UK talent for projects, its good to keep an eye out.

Good luck