I often get the 1805 from London Bridge. I hate it as well, because they tend to announce the platform at 1804 (accually that is early for the 1805.)

However, I noticed, that if you go to Southern's web site and look for live departure boards, it will tell you the platforms for all departing trains for the next hour. The 1805 tends to be the 10, 11, or 8. But if you check the web site before you leave work, you can go straight on to the right platform, without having to wait for the annoucement.

That is unless the 1805 is delayed (as it often is) and it is just easier to take the 1818 (platform 13). I am getting dangerously close to trainspotting now, aren't I. Ugh.