hey there expat...
you have already received some very helpful info from the others
ive been here for one year now and still can remember like yesterday that 3 month mark. i think its the hardest hurdle you have to get over. although i still get homesick i dont get down as bad as i use to. so hang in there...things do get better.
as far as volunteer work goes....i did a lil bit of volunteer work before we moved out of london. like you, i thought it would be a great way to meet people and ultimately give me something to do. unfortunately...it didnt, lol. when i went for the interview there was LOTS of ladies around, all volunters doing various jobs. i thought WOW i hit the joke pot, lol...PEOPLE, new friends, and a social life!!! anway, my first day the director showed my what i needed to do....answer phone (support calls from all over england, wales, ireland), and clerical stuff. at first i thought cool. but after that first day i realized....NO ONE else worked on mondays except for me and the director and she was down the hall so i had no contact with her at all. and on top of that i couldnt understand half the people that were calling in for support (YIKES!!) after a couple of months of really TRYING and forcing myself to even show up...i asked if there were other days i could work (i need people) but there wasnt. anyway, i toughed it out and stayed on until we moved. it was a good experience for me, i reckon and something to put down on my UK resume.
so....let that be a lesson for ya, lol. make sure when u volunter ASK QUESTIONS!!, lol. definitely find out the who, what and when...regarding the job/position.
GOOD LUCK!! and keep us posted