You cannot fly IFR in a Homebuilt PFA/LAA permit aircraft. Some countries require IFR to follow airways, some (France I think) allows VFR on the lower levels of airways.
There is talk of this changing, but don't hold your breath.
You can fly abroad. Some countries require you to get overflight permission for each aircraft individually, some countries have blanket rights. The PFA/LAA website has the list, but most of the countries close to the UK have reciprocal rights, so you can fly there without doing anything.
DJ