There is a difference. Jam in the UK, is what Americans call jelly. Jelly in the UK, is what Americans call "Jell-O".
The main difference, is how to use these words. Consider who you are talking to, to ensure that you make your meaning clear.
If you are talking to a British person and mention jelly, they will think of what Americans call "Jell-O". If you ask for jelly in the UK, you will end up with "Jell-O".
If you mean what Americans call jelly, you will need to use the word jam.
Otherwise, this can lead to a misunderstanding.
These pages explain it: http://dictionary.cambridge.org/dictionary/british/jelly?q=jelly and http://dictionary.cambridge.org/dictionary/british/jam_1?q=jam