Post is delivered Monday to Saturday in the UK. You can choose first class or second class delivery. Second class is the slower, cheaper option. Most first class mail will arrive the following day, but it’s not guaranteed. If you need to send something securely or quickly (like a passport) you can pay a bit extra for special delivery, which insures your item and provides online tracking and proof of delivery.
If you need to find a postcode use the postcode finder service on the Royal Mail website: http://www.royalmail.com/postcode-finder
If you aren’t home to receive your post you can organise to have it redelivered on a specific day for free here: https://www.royalmail.com/eredelivery
If you need anything sent to the UK when you first arrive and you don’t have an address you can use the Poste restante service to deliver items to a local post office to be held until you pick them up: http://www.royalmail.com/delivery/outbound-mail/poste-restante
Find your nearest post office here: http://www.royalmail.com/branch-finder