I'm glad I read this thread because I now know that in the UK (and in particular England) it may be considered rude to go to a funeral if you're not a family member or very close friend of the deceased. I'm constantly amazed at the cultural differences between us even though you're just across the sea.
As others have said, in Ireland it's normal to attend a funeral even if the deceased isn't personally known to you e.g. a colleague's DM or DF. If you're not familiar with this custom, it may sound like rent a crowd but it's considered to be showing respect for the dead person and/or showing support for the bereaved family. People say proudly afterwards, "There was a great turn out", or, "He/she got a great send off".
It's the custom to organize soup and sandwiches (usually in a pub) after the funeral mass. This is in recognition of the fact that some people may have travelled a long distance to get to the funeral and may be in need of sustenance. As the venue is a pub, people often buy drinks and exchange stories about the dead person and speak about him/her fondly. It can be very comforting for the bereaved family to hear others speak of the deceased with such affection and to hear all the stories, especially if the stories are new and they never heard them before. My friend told me that after her DM died, several people came up to her after the funeral to tell her that her DM had helped them out years ago when they were going through a hard time and had given them money or food. Her DM never said anything to her family and they wouldn't have known this except that the grateful recipients came to the funeral to show their respect for this kind lady.
I wouldn't go on to the pub if the deceased person wasn't well known to me. If a colleague's DM or DF died, we would go to the funeral but head straight back to work after the funeral mass.
If your custom is to have a small intimate family gathering for a funeral then I can see that it would be intrusive and rude to have others show up.