Clarifications needed: UK research positions In addition to various US-based positions, I am currently considering (applying for) several research positions at a couple of well-known UK universities. While I researched the topic somewhat, I am not too familiar with specifics of academic system in UK and, thus, would appreciate clarifications.

Questions:


Despite being offered at the same salary level, I assume that a position of researcher is higher in rank, responsibilities and expectations than the one of research assistant. Is my assumption correct universally or it might depend on department or specific research project?
Is employment at UK universities at-will? Specifically, I am interested in the context of (postdoctoral) researcher positions. If the academic employment is at-will, is it ethical to leave early a research position, associated with a multi-year project (for valid reasons and assuming no teaching is involved)? If not, how such issues should be handled gracefully? I don't expect such things to happen, but it is better to have some situational awareness in this regard.
Are there any differences between positions with the titles of researcher or research assistant and the ones with the same titles, but prefixed with "postdoctoral"? (You can answer from both UK and US perspectives or either one, whatever environment you have knowledge about.)
Re the middle point: on anything but a casual labour contract, "at-will" employment does not exist in the UK. You cannot be fired by your employer without a reason, although note that many of the protections do not kick in until you have had a certain number of years in the job, and hence may not apply to fixed term contracts (IANAL). 

From the employee's side, legally, there is nothing to stop you leaving at any time, after the period of notice specified in your contract. That does not mean that it might not have career implications - but I will leave that for more experienced people to discuss.