I know all the pre-A+ 1275 Blocks, you can take 3/8" (~10 mm) off the deck no problem. When they made the Cooper S Blocks, they all had an extra 3/8" 'slab' on the deck. If the block was being machined for a 1071 or 970 Engine, they would machine this off to reduce the block height, only to get the pistons nearer the deck (to maintain CR), further to this, for the 970 engines, they had a longer rod for the same reasons.
From a production (and engineering) point of view, it always seem a daft way to go to me, when all they really needed to do was use 3 different pistons, each with differing pin to crown measurements. Anyways, that was BMC's call.
When they did the 1275 'solid tappet cover' block, that extra 'slab' was maintained. They did also carry this over to the early A+ blocks, however I think in later years, it was thinned.
Alex, I can see a lot of merit in lowering the engine in the subframe, but also a shed load of work to do it! I'd want to look at it in more detail, but I'd probably be inclined to start off with an Auto Subframe, that will get the engine sitting about 1 to 1-1/2" lower off the bat, you can always raise it from there.