1.You will still have a UK driver no. even though you now have a Guernsey licence, so penalty points etc can still apply on your UK record, although a stiffer fine is more likely. Full co-operation on Road Traffic enforcement is still under discussion with Channel Islands, Ireland, IOM and does not yet apply, so endorsement of your Guernsey licence is unlikely.
2. If not domiciled in UK (and no UK address supplied)then a fixed penalty may not be applicable and you may be summonsed. Failure to appear can result in being found guilty in absentia. A fine may then be applied.
3. Failure to pay a fine usually results in a warrant for arrest.
You may wish to consider whether or not you will ever return to UK and whether or not you want something hanging over you on your return (It won't just go away !)
You could try writing to the speed camera agency advising you have no UK licence and no UK address. They may consider that pursuing the matter is more bother than it's worth. Good luck (I hate speed cameras!)