I go to Blues Jams around Essex...the term 'jam' does seem to be used differently here by different people.
The Blues 'Jam nights' that have survived in Essex that I have been to, tend to put random bands together, but the bands usually play reasonably well known blues songs. The thrill is, playing with a new musicians with no rehearsal time, and responding to each other, especially when half the band rarely have heard the song being played. I love it, especially when the song just flies.
I've been going for about three years to these and it has made me a much better live musician. I have had to try and lock in with a huge variety of different drummers.
Now, another jam I go to is more an open mic night, where there are a mix of individual singer songwriters and other musicians. It's also fun, but a different vibe.
Saxybassist: I go to the Hot Hob Jam and 'Blues at the Nags' mainly. Both really well run Jams in Essex.