Clarifications needed: UK research positions
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.