1. (i) No, I don't address anyone as ma'am. I've never met the Queen and or spoken to a senior female officer in the armed forces or security services, so I have never had occasion to call anyone ma'am.
(ii) There's one other form of address which is important to bring in here: it's madam. In the UK, it is 'madam' - not 'ma'am' - which is female equivalent of the civilian use of 'sir'. 'Madam' is the equivalent of the American 'ma'am' , the French 'madame', the Spanish 'señora' and so on, but it is rarely used.
Go into almost any bank, shop or restaurant in the UK, and you won't be addressed as anything (apart from 'love', maybe). We in Britain are more informal than much of Europe and North America, and these polite terms of address don't come naturally to us any more. The only people who'd call you 'madam' are, perhaps, hotel reception staff and airline cabin crew who are obliged to do so. You get the impression that these employees are saying this between gritted teeth and probably hate having to say 'madam' and 'sir'. A police officer will often only start to call someone 'sir' as a forced politeness when the conversation starts to get unpleasant.
2. I live in southern England, where 'mum' is standard. This is not universal in the British Isles, however. In the Midlands, people say and write 'mom' ( like in the US), while you might come across 'mam' in parts of northern England, Scotland and Ireland.