You've probably solved this by now, but here goes:
My experience is that all brake shoes ('holders') of this type (i.e. conventional callipers) are interchangeable. So you can use, for example, Weinmann shoes and brake blocks ('pads').
I think you're correct when you say the hardness of your existing brake blocks is what's stopping you from stopping; I don't think there's anything wrong with the GB callipers that won't be cured by using better brake blocks.
It used to be normal practice to buy just the blocks and put them into the existing shoes, which were open at the back to make this easy. It was necessary to notice which way round the shoes should be fitted, otherwise the brake block would eject itself under braking, and possibly this was the reason why the shoes came to be closed at both ends.
Don't be put off by this - it's easy enough to bend the closing end wall of the shoe and insert a new block in the old way - just be sure that you keep the end that you've bent open at the back when fitting to the calliper, since too much bending will cause metal fatigue with the possibility of the shoe breaking and the block coming out under braking (as above!).