I'd say a GCSE is nearer to JLPT4 in terms of content. (vocab, grammar points, kanji)
I don't think GCSE covers passive constructions or keigo. Although I can't find a syllabus, (they seem to be a secret, or maybe general guides rather than specific points. --The Irish Leaving Cert, an exam similar to GCSE and A level, has much more clearly defined specifications)
A GCSE is a bit more comprehensive than JLPT though as there is an oral exam and you are expected to give written answers rather than chose from multiple choice.
However looking at a sample paper it's much more simple than JLPT as the exam paper is in English and the answers are mostly in English (The new *dumbed down* 2010 specification?). The pass mark is lower than JLPT as well.
Looking at the 2008 1 hour 15 min writing exam paper, if you can pass JLPT4 this should be a doodle.
A levels (obviously) are better and more challenging from what little I've seen. They seem closer to the old JLPT3.
It is interesting to note that according to the Japan Foundation that most non-native teachers of Japanese in the UK are at JLPT2 level (or below).
JLPT and GCSE's test slightly different things really. JLPT is aimed at adults for starters.
Any exam is likely to be a challenge for people sitting it. About 40% of those sitting JLPT 4 don't pass. (some with very low scores, scoring less than 20% in a multiple choice test you've studied for is a feat in itself, that 3 people had less than 1% in 2008 is unbelievable!).
JLPT4 looks easy to me now, but it was a challenge at the time. It is possible to sit it after a year depending on how many classroom hours or hours study you can do.
Starting from scratch you have a lot to cover. To absorb it properly takes time.
The same would be true of a GCSE except the limitations schools put on people sitting exams. (Anyone with physical access to an exam centre and £60 can sit JLPT)
The Japan Foundation has a page about exams in the UK
http://www.jpf.org.uk/language/exams.php
and the only board to offer Japanese, edexcel is here
http://www.edexcel.com/quals/gcse/gcse- ... fault.aspx