im sorry, but first class isnt gaurenteed next day. it gets there when it gets there, the only garenteed post is special delivery which is either 9am or 1pm next day.
also, i never send anything first class, ever, thats just asking for things to be stolen or 'lost' because its not tracked, if its important, it goes recorded, if its expensive its special. and ive never ever had anything go missing. i must say i mostly use special cause its always next day. if something is heavy, i dont use royal mail standard parcels. ill use DHL, mucho cheaper.