A true referendum, such as one associated with a neighbourhood development plan for example, would need to be run by your elections office rather than the parish and I'd suggest it's too late to add it to the diary for the general election on 4th July. If this is just the parish asking residents for their opinion, you wouldn't be able to do that through the polling station in conjunction with the general election and probably couldn't just sit someone outside the polling station to canvas residents as there are rules about who can and can't be within a certain distance from the polling booths.