A3 cheapest place to get printing done

Last time I did something like that I used Ryman & uploaded & got it delivered.

Not the cheapest but convenient & delivered to my door without walking around places searching for the best price & waiting around for it to be done or having to come back the next day.

My local library prints A4 for 10p per sheet. I'm uncertain whether your local library offers an A3 service. Some estate agencies/local supermarkets/solicitors/etc might do it - failing them any local "prontoprint" might be worth a try.