Congrats on completing your screenplay.
Take a break, then get to work on another script. This is the standard answer. If you are able to get some interest from agents, production companies, studio exectutives, one the first questions they are likely to ask is, "Have you got anything else?"
Good info from an industry insider on submitting to agents, (you've got to have several samples, they all should be first rate).
http://filmdiva.blogspot.com/2006/06/inserting-foot-in-door.html
Another answer is, find or start a writers group, get feedback on your script and revise it. Along these lines, you could find a good screenwriting class, either on line or in person.
Organizing a reading with actors or friends can also be a very good way to get a fresh take on your script. Videotaping the reading and then going over the tape usually helps too.
You can also enter your script into contests. Since you're over there (from my POV) you may want to take advantage of the contests the BBC has for radio and TV drama.
There are also sites where you can post your script and get feedback, Triggerstreet et al. Some people like these, some people despise them.
You can get feedback on your pages in another section on this board, and on DoneDealpro too. Take everything with a grain of salt, and keep writing.
