Yes the 5mm UK board is too heavy to use without modifying the plans. I just buy the speedbuild kits, it’s easier and faster. I have built three planes out of UK 5mm, all were hopeless flyers without extending the nose by 50-75mm to counter the extra mass, plus lighter is always better.
FT foamboard is about £2 per sheet from Model Shop Leeds and Sussex Model Centre plus a few other stockists. If you are scratch building that’s what you will want.