The reason for using pencil is largely an historical one. Before the Electoral Reform Acts of the 19th C, people would vote in ink, by signing their names on the ballot paper. With the advent of the secret ballot this was replaced by a cross (mainly to allow the illiterate to vote). At the time fountain pens were in their infancy and ball-point pens had not even been thought of. The only alternative would have been an inkwell with a nibbed pen. It would have not taken too much imagination for someone to go into a polling station and pour glue or sand into the inkwells, in the secrecy of the polling booth, thus creating electoral chaos.