I know what you mean, I am planning a auto g/b ATF change myslef, not easy with central London street parking.. and you need 4 axle stands to keep the car level while checking the oil.
My concern about working under the car while parked in the street is that I could have my feet run over by a passing car, or that a car trying to park behind/infront will bump my car (as they do) and knock it off the stands - while I'm under it...
As for the sump, it is best to drop both sumps, but if you need to do this in a hurry you could use a 500cc syringe and replace (most) of the ATF through the filler hole. But you will not get rid of sludge and fine metal bits, you will not be able to clean or inspect the filter, and you will not be able to get out the ATF from the small sump...
With regards to 'sealed for life', this is not technically un-true as Vx only specifies auto g/b ATF change under 'extreme conditions', i.e. police cars and taxis, otherwise from Vx point of view the ATF is for life. This is of course false economy, but I can't blame the garages for looking it up in Autodata and seeing that there is no change interval specified for the ATF.