I am also with O2 in the UK and their reply doesn't surprise me in the slightest. However I am already running Jelly Bean on my SGS2, well the leaked 4.1.2 release anyway.
You should read the thread "Jelly Bean firmware unofficial release" and install it from there. It's the first time I had tried anything like that with my phone and it went without a hitch, all done and dusted in less than 30 minutes. I highly recommend that you do it as it's like getting a whole new phone, an S3 in a smaller package!