.... Should we bring our Maytag units from storage in the USA?
Probably a bad idea
Can it / they be adapted to UK electrical requirements?
They should run OK off a suitable
power transformer, though the motors might run slow as UK electricity is 50Hz, (as opposed to 60Hz in the US). Even if the motors could be replaced with 50Hz/ 220V motors, you would probably find that it would cost more than the machines cost you to buy in the first place.
If we don't get a place that can accomodate them, is there a market for American full sized appliances, such as these in London?
You can safely assume that there is no market for your appliances in London, ... because they are larger than British appliances and can't be plugged directly into a standard British electrical supply.
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I think that you will find that appliances in Britain are the same size as those in France (and Germany, Italy
etc.), much smaller than US ones,
unless you buy commercial machines.
You will most likely find that if you bring your Maytag appliances the only place for them is in the garage as most British homes don't have a laundry/ utility room, nor a basement, and so laundry appliances are often kept in the kitchen.