(UK) Smallest outdoor unit for split system?

We're looking to install split air conditioning systems in our offices, which are in a listed building. This means that the whole installation must be invisible from the street. We have a few balconies and parapets where we might put the outdoor units, but they would need to be small. Can anyone point me to the smallest (specifically, the least high) outdoor units available? Each would be serving a one- or two-person office, and would need to be around 2-4KW capacity.
A related question (and showing my complete ignorance of the subject 8-) can you mount an external unit on its back to reduce its height?
Also, how much clearance do you typically need on front of and behind these units for airflow? Our pitched roof comes down into a gully with a low wall at the front, so if we were to put units in there they'd be down in the gully, venting at the tiled roof or at the wall - is likely to be a problem?
(I know the real answer is to get an engineer in to look at the building, but I'd like to get a feel for what ought to be possible before we do that.)
Thanks for any advice you can give!
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