Perhaps the UK henges were trading centres at some point in their life, but in Ireland (the only other places they exist I believe) I don't think it fits the bill.

Take Forenaghts Great where I was yesterday:
http://www.themodernantiquarian.com/site/6219

Excavations here found a network of ritual pits and a couple of cists, one of these at the base of the longstone itself. There are 4 other (very) longstones in the area, none of them in henges and at least two mark burials.

The east and west entrances to henges do not fit a trading post theory for me. These mark the equinox and thus are ritual. A trading post would probably have four entrances at the cardinal points, showing through great symbolism that 'all were welcome' no matter where they came from.

The henge at Dowth is in the midst of a passage tomb cemetery. The one below Newgrange similarly. The Giant's Ring near Belfast encloses a passage tomb.

Avebury is a henge. Does that look like a trading post to you?

I hate to use the 'R' word too much, but henges must have had a large ritual context. Admittedly, that ritual <i>could</i> have been to do with trading, but I personally don't see it being so.