Shops are great, especially when you're getting started because they can get you up and running and troubleshoot if you're having problems, but once you know what you're looking for, they simply can't compete on price. What tank are you using? If you google it, you'll almost definitely get a cheaper deal for coils online.
Eliquid (which contains no oil!) is mostly made of propylene glycol (PG) and vegetable glycerine (VG).
PG: carries flavours better, provides more 'throat hit', but less visible vapour and some people are a bit sensitive to it.
VG: more visible vapour, less likely to give you a sore throat but will gunk up your coil quicker.
Most of the cheaper stuff contains a higher proportion of PG but both of these ingredients are very cheap and you should be able to find liquid with high VG (or even 100% VG) for less than £6/10ml.
Come on over to the stop smoking topic and say hello, there are lots of knowledgeable vapers who can give you some recommendations (I'm no good, I bulk-buy high PG liquid direct from China
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