They have a basic check of documents before you Pay at Croydon which was much like what the checking of the documents was like before sending off for the Nationalty application.
If you have doubts about your paperwork or sending stuff in the post etc I would opt for the one day. Was well worth the extra especially in the lack of waiting for days/weeks. Took very little time (we spent more time in the shopping centre across the road) and of course meant you could take everything but the kitchen sink and explain paperwork etc with the person in front of you rather than write cover letters etc if there was some sort of issue..
Some people who took the bare minimum paperwork seemed to have more problems and took longer than us I do think they are swayed if they see that you have the normal household paperwork in bulk that everything is in order..
