In reality - it doesn't really matter tbh
you can save shitloads DIY or buying retail juice
(edited my juice is cheaper crap)
In reality say £100 a month on average = £1,200 a year
as I go nutz on shiny shit now & then
Say half of that is UK spent (£600, so £100 VAT paid)
Other half is from overseas (no VAT)
When I smoked £10 a day = £3,650 > £2,500 VAT/Duty
That is a nice feeling as I puff away in my own little happy place
The real saving is your own health, your wallet & lower tax contribution
(in that order I reckon)