Thank you for contacting AlterHéros. First of all, I must commend you on your acceptance of yourself, especially faced with the loss of your friends. It takes someone courageous to affirm who they are even in the face of adversity. I'd also like to say that the friends who have left you because of who you are don't reflect any error on your part, their ignorance is their loss. That being said, I think you'll find that your friendships will be stronger with those friends who remained with you or your future friends that will accept you for who you are. Secondly, your sexuality is merely a small part of you, and if your friends don't see that you are the same person it's their loss, though I understand the loneliness you must be feeling.
As AlterHéros is an organisation based in Montreal, Canada, the only resource I have to help you find youth groups is the internet. I have found a few websites that list the different youth groups that exist in London and their contact information:
The first website also has a lot of extra information on how to deal with your sexuality and the different resources available to you. You may have to verify if some of these youth groups are still operational, but there are a great number of them, so you should be able to find one that suits your needs.
I also recommend you look for other online forums and gay communities. There are many youths on the internet that communicate with each other through such venues. While it may not be face to face communication it is still quite helpful to quell those feelings of loneliness. It has been very helpful to me in the past and I hope it may lend you some comfort that you are not alone and that there is quite a big network of gay and bisexual youth on the internet.
I wish you the best of luck and if you have any more questions feel free to contact us again,