Don't bother with branded versions, the ones made to look like ciggies, generally they deliver an expensive and frustrating experience.
Buy a good Ego type battery (2 so you can use one while t'other charges) and a couple of CE4 type clearomisers. These screw onto the top of the battery and contain the e-liquid.
Google these terms for UK mail order suppliers.
E-liquid comes in many flavours and strengths, if you smoke 20 or more cigs a day you will need to start on at least 24Mg E-liquid. If you are able to entirely swap your ciggies for e-cig use you will find you can reduce the liquid Mg strength over time.
There is NO eliquid tobacco flavour that actualy tastes like a ciggie does. After a fairly short time this will reveal itself as a good thing Go to a shop that has samples you can try, you will find something that you like.
I started vapeing in May 2012 from a 45 year laterly 30 a day smoking habit. Never bought another pack of cigs since its arrival, gradually weaned myself off the nicotine content using it.
Running costs can be as low as £20 a year for the hardware and £5 to £10 a week for the e-liquid. Less if you shop wisely
It worked for me, your mileage may vary.