The Spanish surgeon's plan is not here to refer to. But that's OK because I think I can address some of your concerns as to the differences regarding the EXTENT he wants to move things that are more than the UK surgeon.
1: There is a relationship between the amount of maxillary advancement and increased tooth show. For every 'X'mm of maxillary advancement, there will be 'Y' mm of increased tooth show.
Y will be less than X. Although I've heard, on this board, something to the effect of; For every X mm of max advancement there will be Y mm of increased tooth show where an 'exact' mm measure of Y was given for each X, I have no reason to believe that would be the same in each individual. However, there is still a (general) relationship which can be stated as; 'For every X mm of max advancement, there will be Y mm of increased tooth show'. However, what 'X' is going to be for 'Y' to be what it's going to be will be specific for each individual and will depend on the orientation of one's maxillary plane and is something the surgeon figures out, especially those with the high tech planning programs.
2: What to glean from the above is although the Spanish surgeon is impacting MORE than the UK surgeon, he's also ADVANCING more so that there is enough tooth show. So, due to the relationship as stated above, one can't conclude that just because he's impacting more than the UK guy you'd be left with that much less of tooth show that the UK doc would give.
3: As to having a long face where the forehead, in it's own right is 'absolutely' long (high forehead), it is not a good (aesthetic) idea to stick to the 'rule of 1/3rds' in any attempt to 'balance' the lower 2/3rds with the upper 1/3rd (the high forehead) because it resolves to the act of trying to balance the lower 2/3rds with an IMBALANCE the upper 1/3rd has as to being too long in its own right. So, to actually want the lower 2/3ds to 'balance' with an IMBALANCE to the upper 1/3rd resolves to wanting to be STILL left with an overall LONG FACE. It is preferable to balance the lower 2/3rds (shorten them, advance them) and be left with harmony/balance to the mid and lower 3rds and just keep the high forehead or elect, later down the line to bring that into balance with the lower 2/3rds which can be done by bangs or a hairline lowering procedure. Besides, neither Leonardo DaVinci nor Durer (who came up with the rule of 1/3rds) ever said that equal 1/3rds equaled ideal balance. Instead, it was a DRAWING guideline for drawing a face. So, having equal 1/3rds is PART of the 'formula' but NOT all of it as it relates to aesthetic balance and especially not if upper '1/3rd' is 'long' or 'too long'. Trust me, I studied Fine Arts at Harvard. Also a high forehead can be considered associated with high intelligence, so you could keep it and need not fret that it remains relatively longer looking due to aesthetically balancing the lower 2/3rds IN THEIR OWN RIGHT and not with reference to balancing with forehead.
4: As to shaving part of the chin down, that might not translate into a 'bad thing' or the act of lopping of the base of the chin to make it 'too short'. The relationship at play is that with the BSSO, the chin point (pogonian) moves 'forward' along with the mandibular advancement. The 'forward' advancement is going to be outward AND downward because the mandibular plane is oriented on a downward diagonal (even after it's inclination is made less from the impaction). Technically speaking a BSSO alone can make the chin cast BOTH longer and of course, more outward. However, just like an advancement genio can be ADDED to a BSSO if the BSSO itself, cant achieve an 'ideal' balance as to chin pronouncement (the BSSO also has to balance the BITE), any EXCESS to either the outward displacement of the chin point or the downward displacement to the base of chin can be SUBTRACTED by shaving down where the excess is. It could be to the front of the chin or to the base of the chin. It does not necessarily 'translate' to lopping off the base of the chin to make it 'too short'
Although these types of relationships don't predict how you are going to look, they should assuage some of your concerns that (to me) reflect you didn't realize those relationships existed.
5: As to the UK surgeons displacement diagram, that looks like a very conservative yet an aesthtically improving one also. My reference is bimax plus genio contour displacement diagram (the 4th one in the series). With a contour displacement diagram, you don't have to concern yourself with the numbers and arrows on it because the DISPLACEMENT CONTOUR of the SOFT TISSUE PROFILE is DRAWN in which is the brown contour. So, the brown contour is a great improvement over the present contour (the green one). However, SEEING that the brown soft tissue contour is an IMPROVED one is a matter of how well you can see aesthetic relationships.