Congradualtions on reproducing London water.
When I toured Fullers, they had a pallet stacked with of bags of gypsum. Not to make plaster...
The brewers adjust their water, you need to think about what they are trying to achieve.
As I was just going through my final (probably) brewing salt calculations for this recipe this comment popped back into my head. Per advice above I decided to just dial back my additions to add a little gypsum to bump up the Ca a little. However your comment made me wonder why Fullers would do that. And my conclusion is that in addition to Ca they're bumping up their SO4.
According to Palmer:
The sulfate ion also combines with Ca and Mg to contribute to permanent hardness. It accentuates hop bitterness, making the bitterness seem drier, more crisp.
And per the BJCP style guidelines:
...Medium-dry to dry finish (particularly if sulfate water is used)...
Is this what you were referring to? It seems to make sense if brewing to style. I'm sticking to just gypsum for this brew so as to not overdo things but what level of Ca and SO4 would you recommend for the style? I realize I'm going a little overboard here, and I'm not shooting for perfection by any means, just a good beer. But I'm just curious as to the effects of the SO4 on hop bitterness.