I'm not sure if sink plumbing is the same in the UK as it is in the US, but I can tell you that I plumbed in under my sink. If the standards on either side of the pond are similar, you disconnect the hose going from the valve coming out of the wall and the faucet and insert a valve like this one here:
John Guest Angle Stop Valve - Bulk Reef Supply
The harder part is going to be what you do with the waste water line. I guess you could simply run that line through the drain in your sink when you're actually producing water. I used a drain saddel clamp like the one in the link below for mine, but it does require drilling a small hole in the drain pipe to plumb it in.
Drain Saddle Clamp - 1/4" Push Connect - Bulk Reef Supply
As far as brand goes, I'm not sure what's available there, but I've learned the component pieces of an RODI
unit are pretty much all the same. Some companies do a better job with their instructions, assembly, and customer service though. I do advise you get one with a DI
section that is vertical and refillable. The vertical ones are more efficient and being refillabel will save you money in the long run.