Well, let's see - that price for 30ml looks very close to trade, Heed. Add in shipping (at least $45 via TNT express - remember this is a business buying), Duty and VAT and it may come into the UK business at, say, £5.50 a 30ml bottle. Then, it may have to have CHIP caps put on (the chinese tend not to), be relabelled to conform to UK Trading Standards directives (again, what comes out of China doesn't), and that takes time, and time costs. You may find the business paying, then, around £6 a bottle, so a retail price of £12 odd isn't profiteering, and £15 is not outrageous.
You also have to factor in the cost of actually paying the bill across to China - I can tell you now that Barclays wants £45 to make a payment to a Chinese bank. I can't see that any of the more recognised outlets in the UK is making a killing on e-liquid.
And I can pretty much guarantee that the UK made juices will be far more costly to manufacture than buying in from China - again EH, TS and lord knows what other agencies get involved and apply costly standards to the premises etc...
In short, I don't think UK sellers are ripping their customers off - HMG might be, but I don't think it's down to the retailers.