You could always move over here, Less... Lancaster City Council say it's OK to put window envelopes in the recycling. Or you could pack yours up and send them to me...
Margaret, I wouldn't be too sure - plastic can masquerade as all sorts of things these days and even those that look like tracing paper could have some plastic content. The only way to be sure would be to contact the envelope manufacturers and I think HBC should be a bit more specific too.