What is the 11 digit post code for the UK?

In the UK, a postcode is equivalent to a US Zip Code.

They are a mix of letters and numbers between 5 and 7 digits in length, with two halves separated space in the middle. examples: W12 7RJ, SY12 1AD, B1 1BT, TN9 7BT

A UK postcode would never be 11 digits long - and would be different depending on the address.

So, perhaps what you are thinking of is not actually a postcode? You might need to explain a bit more or give an example.