You dont use the post code. You need the international weather ccode.
Here's what I do - there are easier ways, I am sure.
go to weather.com - local weather forecasts, radar and reports from The Weather Channel
from a PC
Type the name of your city/town into weather.com
For example, I typed in Liverpool - after finding Liverpool, UK, the weather for Liverpool came up.
The browser address line will now have the location for Liverpool in it - UKXX0083
Then put UKXX0083 into BBWeather.