Letters are addressed:
Mr. Walter C. Brown (recipient)
49 Featherstone Street (street name)
LONDON (town/city)
EC1Y 8SY (postal code)
UNITED KINGDOM (country name)
Note that the return address is usually on the back of the envelope.
1st class mail ensures one day delivery starting at 46 pence.
Second class mail starts at 36 pence with 3 day delivery.
Packages start at 4.21 GBP with 2 day delivery.
More pricing options found here.
There are over 500 main post offices and a further 17,000 branch or sub-post offices which are operated on a franchise or agency basis. The Royal Mail offers a directory of Post Offices.
When you move home, whether temporarily or permanently, Home Redirections service will forward mail from your old address to the new one. Mail can be redirected for anything from one month to two years. To use the service, go to the Royal Mail site.