Am I right in saying that you would use flex (as opposed to flat cable) if the cable is likely to be held, manipulated, moved around, outside etc?
There are various factors to consider.
1. T&E is not generally considered suitable for outdoor use because it's not UV stable and becase it's flat design means it doesn't seal well with glands. There doesn't seem to be an "outdoor T&E" among the regular UK cable types so an outdoor rated flex often ends up being the easiest options for outdoor work where armouring is not required.
2. Flex is flexible and designed to stand being moved around, the flip side of this though is that the flexibilty means you often need more cable restraint if you *don't* want it to move. T&E is better at staying where you put it.
3. Flex is in my opinion much more of a pain to strip, in particular it's far more of a pain to strip the sheath without damaging the inner insulation. T&E with it's uninsulated earth wire in the middle gives you a safe place to aim with your knife.
4. The fine stranded conductors in flex are prone to fuzzies when inserting into terminals. This can be a particular pain when doing wiring in awkward locations or when dismantling and reassmbling wiring. Some people use ferrules to avoid this but ferrules have their own issues. On the flip side solid wire can be prone to breaking if the core is "nicked" during stripping.
5. It's considered bad practice to mix solid wire with flex in the same terminal. So if you are going to use both solid wire and flex you need to think about the transitions between the two.
6. Flex looks better if the wiring will be exposed.
I plan to fit a loft light and a part of me feels like I should be using Flex cable?
It does depend a bit on what type of light you are going for. Some fittings are more suitable for T&E, some are more suitable for flex, and some come with a pre-wire non-removable flex tail that will need to be connected to your fixed wiring somehow.
