You need
Nic...if you use nicotine, I'm guessing you do if ya run out to Asda lol
PG - Propylene Glycol
VG - Vegetable Glycerin
That is your base liquid, VG is thicker, which we use for sub ohm vaping, I vape a 30PG 70VG Mix
Then you add a percentage of flavor, maybe 10% up to 20% depending on your flavors.
You can use an e-liquid calculator to make your juice
I use this eJuice Me Up - Best eJuice Calculator but there are many more....
Best place for Nic, PG and VG in the UK
Everything Liquid - EP/USP Liquid Nicotine, PG/VG, Flavouring
There Nic strength is 72mg
As for flavors, best place in the UK
Vapable - Incredible Value E Cigs, E Liquid and Accessories
Looks for Capella, TPA, Flavor Art ...
Or if you just want an e-liquid ready to vape and you like custard then the only place, not only in the UK but in the world has to be Isle Of Custard - Golden Pirate E-juices
Hope that helps
