I have a GPSMAP 62S with 1:50K OS Discoverer maps. When searching for a postcode the FIND will not let you enter a complete UK postcode.
When you FIND > ADDRESSES > Enter the Country > Enter City/Postal Code it will not allow you to enter more than 3 characters before it prompts for house number and then street name.
UK postcodes can be upto 7 char long but it will not allow you to enter more than 3 of a valid postcode. So you need to know the street / road name as well. This isn't always known. Also it means a lot more characters to enter rather than the 5-7 of a post code.
It would be very useful if it you could enter a complete postcode like you can in most other gps devices such as Satmap10.