The actual parade is by invite - organisers contact regimental assns, RBL regions etc with quota's. Not sure what would happen if you tagged on at the end. Can't see even the Met making a big deal but outside my experience.
Personally, I prefer the local one. If I went to Lunnen and saw Livingstone I'd fall out and shoot the b*st*rd
You will need a ticket to gain access on to Horse Guards Parade - think of the security nightmare. Tickets available from The Royal British Legion, which you should all join if you are not already members. The membership is in a sad state of decline and needs some fresh blood. I collected for the Poppy Appeal for the first time this year, and it was a fascinating experience confronting the Great(?) British public and extracting money from them both on their doorsteps and on street corners. A real eye opener.
The Remembrance (note the spellling) Parade is a wonderful occasion if you get the chance to 'get fell in'.