What advantages do I have buying it separately? Money saving?
Having a look on darkstar, you can now only buy shots. Are they nicotine, or PG&VG mixed 'shots' so they just need added to a concentrate or do you still need to add VG+PG. It's all a bit confusing after this new law.
Normally 3mg.
For the most part, it's money saving and you're not tied to a certain ratio (50/50, 60/40 etc.) - you can mix it at whatever ratio you like.
A 70/30 3mg base mix will end up being 60/40 or 50/50 once you've added the concentrates to it (depending on the percentage of concentrate).
The nicotine shots are purely nicotine, TPD law states max they can be are 10ml @ 20mg strength iirc (glad I bought 2 litres @ 72mg before it set in!).
Darkstar do their 'Bottle Shots' which have the concentrate in at 20%. To this you'll add however much nicotine/PG/VG you need for say a 3mg mix.
Quite an in-depth guide on their FAQ - https://www.darkstarvapour.co.uk/faq
Ultimately it won't be as cost effective as using a 72mg base, but you'll be in control of the ratio.