The one on the fifth floor is only signed from downstairs as 'toilets' so tends to be empty and is much more spacious. No windows though. But I'm pretty sure there's plug sockets.
The fourth floor one has a long thin room for breastfeeding so you can feel a bit like on a production line in the row of chairs, and sometimes can't get a seat, but if there's only a couple of you in there it's nice.
Mothercare on Oxford St usually has the loos/rooms shut because of vandalism/floods/being used for storage/it's Tuesday.
Selfridges have a 'mother and baby' room - up and down small flights of stairs! Not great if you have a pushchair. I ended up feeding ds on some lovely sofas designed for gardens, looked at the price tag, and just prayed I wouldn't get milk on them...