UK guns are notoriously slow loading. so you'll want to class as reload.
depending on what you role for your gunners in the first place, or which ones you purchased if buying elite gunners, you may want to try for say a 11/10 (acc/rel), unless you really want to sit there all day (as some people, sadly, do), to roll better. then class as reloads. that'll give you a leg up on accuracy, but when you class as reloading gunners, you'll get a boost there as well.
the US, unlike the UK, you actually can switch by choosing one type, then the other at the next classing. unfortunately, the British aren't so lucky.