Hello, Brit married to an American here! To say 'I'm ill' is the same as to say 'I'm sick' (which is more American but it's becoming common in Britain) and depending on the region, you may also hear 'poorly' as in 'she stayed home because she's poorly' - this is common around my hometown in Yorkshire, northern England. To say 'I feel sick' means that you have nausea or an upset stomach and 'I feel ill' could be any other kind of illness, such as fevers. Thanks to globalisation, they're becoming interchangeable.

Hi Kseniia No, it's still "sick pay" and "calling in sick" in standard British English. "Sick" means "ill" but also means vomiting, as in "She's being sick", or "I feel sick". Steve Ridley

Does the "change 'sick' into 'ill'" scheme work here? No, it doesn't. British English uses 'sick' for all collocations to do with employment - sick pay, sick note, sick day, sick leave, call in sick ( even 'pulling a sickie'). The word 'ill' started life as an adverb - the opposite of well - and tends only to be used in a predicative form ( apart from archaic collocations such as 'an ill wind', meaning bad). So, we do use 'sick' in the sense of generally unwell if it comes before the noun, both in the above collocations and also in other adjective+noun combinations e.g. 'He cares for his sick wife'. But in all other cases, 'sick' is generally understood to mean nauseous - actually throwing up or feeling like you're going to throw up. If you say 'I feel sick' to a BrE speaker, someone will hand you a convenient bowl, and if you say 'I was sick last night', we'd understand that you threw up. Here's a question I asked a while ago: https://www.italki.com/question/412100

If I say 'I feel sick' in the UK people will understand that I feel like I'm going to throw up. In addition to 'ill' another option which hasn't yet mentioned is 'I don't feel well' which is very common. It's all rather confusing, from what I understand if you say 'I am ill' in the US they will take that to mean you have a more serious ailment, rather than simply feeling unwell. It could potentially mean that in the UK but that wouldn't be the most common usage. Calling in sick is colloquially called 'pulling a sickie' in this country.