The stuff I have is 206.6g per A1 sheet (594 x 841mm)
Just measured a Flite Test board for comparison and it's 111.6g / 20x30" (510 x 755mm as measured)
So the UK stuff I've been using is 413 gsm vs 290 gsm for the waterproof Flite Test board; which is some 42% heavier.
NB. I've recently been using
this seller on Ebay. Their boards are about as cheap as I can get them without having to buy huge quantities (pallet). I can also recommend them as one of my orders got bent double by the carrier and they happily replaced them with no fuss.
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I'm with you there.
I've had good success marking out all the cut lines using a
slightly modified cheap vinyl cutter. The cutter's knife cuts through the top paper layer leaving a nice deep score line for a knife to follow (you don't even need a ruler).
With a fairly simple model (eg a Versa wing), using this method I can go from a blank foam board to being in the air in about an hour or so.
I also tend to scale up my models by 10% or so as the UK board I use is 5.4mm vs 4.8mm for Flite Test foam. This makes A&B folds fit much better and allows any tabs to fit together properly.
The extra size also makes some allowance for the additional weight of the denser & thicker foam.
The fact that A1 is pretty much exactly 10% larger in both dimensions than FT boards works out nicely as well.
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Whereabouts in the UK are you?
If you're nearby I'd be happy to run some plans through the cutter for you.