In advance these are probably quite short (and blunt) but user experience/user interface is part of what I do day to day
On the homepage there is a huge amount of blank space before the hero image, from the primary navigation down there is a good inch to 2 of vertical space that is just blank, you could reduce both by half an inch and then on getting to the homepage you can see the details about the postage (2nd thing I look for after ideas on pricing)
It would help to see some prices on the images scrolling, the first image is redundant, I know where I am (on your site)
The resizing also makes the site move and is a bit jarring when looking at content further down (if you're html savvy you want a min-height on the container for your scrolling images)
There is a big whoopsie on the homepage, I clicked the link advertising the fruit e-liquids and get taken to....
http://mellowpens.co.uk/First Slide
a 404 (page not found) error page. I'd get that pointing to the right place soon
http://mellowpens.co.uk/product-category/e-liquids/
There's a huge gap there in the page, stick something in there like best sellers, new products etc
Same concrens on the social part, if I'd put those on the site it'd be on the home page or off to the right so you can still use the site without it loading down at the bottom (and maybe put a smaller marquee in there too, it's 2/3 full and the empty space could be much better used)
On a product page
http://mellowpens.co.uk/product/blackberry-e-liquid/
I don't need/want to see a link to follow you on twitter when I'm about to buy something, it's an unnecessary distraction (the FB like is good though)
http://mellowpens.co.uk/information/
This comes under shop, I think it's in the wrong place (maybe under the about section?)
http://mellowpens.co.uk/contact-us-2/
Might want to correct that URL if Wordpress lets you, the -2 at the end is a bit messy from looking at the url
BUY AN SSL CERTIFICATE!
I'll never (not to mention tell everyone I know not to) buy anything online from a shop that does not have a valid SSL certificate. It's what keeps my information safe when it is sent to you through the internet. I know you're not taking money directly but you're collecting personal details (including account passwords which people do reuse on a regular basis). They aren't expensive (£10/year ish) and can save a LOT of headaches later on when you collect any personal information
As far as trading in the UK, you need more than just a contact us form, you need at least a name, postal address and email address. You can get sample T+Cs and details on things like that quite easily
http://www.seqlegal.com/blog/selling-online-and-law-part-4
On a whole I like the site, it's fairly simple, the colours are nice and plain with nothing too garish, the content is pretty straight forward and to the point. The stock selection could do with being a bit more varied but that will happen over time. I'd like more info on the liquids though, are they mixed yourself or bought in? If so what brands? The nic levels seem a little bunched with nothing around the mid but that happens with some brands due to popularity so doesn't bother me that much.
The text could use some work to sit down and plan out what it is you want to say, some of it is a bit confusing ("Our e liquids are made to order as this enables us to offer more personalisation without having to home mix", they aren't really more personal, more professional and consistent maybe (though I'm sure some on here would disagree)) and some of it just seems to tail off mid sentence ("Keep up to date with our offers and new products or just say hello", How? Where? What offers? I wouldn't go on an e-commerce site to say hi). It would be good to know more about a site and why they do it (if you vape yourself say so!)
Apologies if that's a bit of a wall-of-text, but if you have questions I'd be happy to expand on my thoughts behind the comments