Section B (Address) is not in the standard 5-line Royal Mail format. The standard form does not have county, but should have post code.
County is NOT used because post is not always delivered from a post town in a different county. For example, Kinsham, Worcestershire has a postal address of Kinsham, Tewkesbury and a GL (Gloucestershire) post code.
The most usual 5-line address in the Royal Mail system is..
B11: Address Line 1: House name or number or apartment/flat number and name of apartment block
B12: Address Line 2: Street name
B13: Address Line 3: District or Post Town
B14: Address Line 4: Post Town if not cited in B13
B15: Post Code
You will then need a B16 for country.
In the US the descriptions are slightly different
B11 and B12 are OK, although apartment numbers are written differently
UK B11: Apartment 12, Balham Park Mansions
US B11: Balham Park Mansions #12
B13 in US would always be City
B14 in US would always be State
B15 in US would always be Zip Code.
Having said that, the Royal Mail is not consistent when it comes to apartments,
In some cases, the apartment number is in B11, apartment block name in B12, and all other fields shifted down a line.