Thanks RedVee.

When I go to the Czech Republic for work, I can change my seat to 22F on a A321 as there is no 21F in front of it (but there is a 21D and E) and I done this for free.
From reading FT, it must because my work travel agent must have some sort of corporate deal with BA.
It was just a surprise to get an email from BA saying we can choose our seats for our flight to Jersey in April but then going online to find we'll get charged for it.