Depends on what you want to cover – for example, the last large Stop the War demo was held in Manchester and the G8 was in Edinburgh.
Most large national demos are advertised across the UK as particular groups organise coaches to get people to London.
Unless you have a specific concern though, I dunno if a 4 hour train journey is worth the effort. Most demos I attend (and not many nowadays) are fairly short notice affairs and are just a tube ride away. If the demo is significant (like the G8 in Edinburgh), or something I’m interested in, I’d make the trip, but not really otherwise, unless you can conjure up a commission and then have a day in London into the bargain.
It really depends on what you want to cover. IndyMedia is pretty good, although if you wanted to cover something that wasn’t particularly ‘anticapitalist’, like say, a pro-Israeli demo about the Lebanon, or a picket outside the Russian Embassy about the Alexander Litvinenko Affair, you’d need to hunt elsewhere for the info.
There’s no real database which covers them all as far as I know.