Hi Royi, very interesting question, and a lovely spot to live in, magnificent parkland all around you there I know it well, now those planners knew what they were about!!
Please don't try reducing evapotranspiration by reducng the live foliage of your trees by anything like 50% let alone 70%...
The effect on even healthy trees of reductions above 25% in the live canopy is very detrimental long term.
If you're thinking of
Robinia pseudoacacia it's actually a native of the Appalachian Mountains (below 3500 feet) but it does have a deservedly good reputation as being tolerant of a wide range of soils and conditions, yes it will tolerate drier conditions than its place of origin, however you would need to get the tree to a size able to sustain its system through such poor times, ie established 5-7yrs.
The soils you have in that area of London are fairly reactive clays and you need to bear in mind whatever you plant will change the soil moisture levels and have an impact on both soil plasticity and the potential for ground heave or subsidence..basically ensure the planting is not too close to your foundations, ensure you provide irrigation to the root system, even if its a bucket every week, but maintain that watering pattern. Fluctuations in soil moisture levels are not good in any situation particularly not with reactive clays.
There is no reason why you can't grow beautiful successful trees in your part of central London, but common sense when it comes to the eventual size of the tree and the way you intend to water it are essential.
If fruit trees are the way you decide to go remember that the Bay tree,
Laurus nobilis is an excellent companion for citrus and is extremely drought tolerant when established..the leaves aren't half bad either.
For more UK drought resistant plants check out
http://www.rhs.org.uk/advice/profile..._resistant.asp
SF