I did extensive 'research' on this over the last few months, hozelock make nice systems but they are limited and expensive. Far easier to make your own to exactly the specification you require.
this to attatch to outside tap (if you plan on running several lines
13mm irrigation hose
then one of these kits (can be picked up in wilko or cheapo shops for a few quid normally.
then u need a basic watering computer, the simple ones are about 30 quid.
here's a vid of how to set it up
You can add all sorts of different types of drippers, depending on what style of watering you want to achieve.
Yes expensive if you buy from B&Q, but you can source lots of this stuff cheaper around the net. You also will be able to tailor the system precisely to your needs. If you look at professional systems, you will notice they tend to always use 13mm hose rather than 4 mm. This is because the 4mm can block much easier, if you use the 13mm as the 'feeder' pipe then individual 4mm piping, you have better control and also if there is a blockage, it's only on one dripper. The 13 mm pipe can also be used directly on beds with drippers inserted straight into the 13mm pipe.
Hope that gives you some food for thought.