UK: Can i use my iPhone 5 sim in an iPhone 6 phone?

UK: Can i use my iPhone 5 sim in an iPhone 6 phone? - 1

Added (1). I live in the UK and am looking into getting an i phone 6 - i have an iPhone 5 sim with O2 i was wondering if i ordered an iPhone 6 (Unlocked) from apple or amazon for example could i just put my current sim in it? And if so would i lose out on anything by doing it this way. Like for example would i not have 4G as i did not have that with my iPhone 5 or is that nothing to do with the sim? Also is there any other downsides to this approach?

Yes, assuming the phone is either locked to the same network you are on on now or unlocked.

Yes, no problem.