Tracks closest to you will be Brands Hatch, Dunsfold Park (top gear track), Abingdon & Lydden Hill.
Out of those Brands Hatch is the best track IMO, however Abingdon would be the best if you have not done a day before.
Abingdon isn't really a circuit, its a airfield which is made into a circuit using cones, this means lots of runoff which is great for getting used to the cars limits.
These days are also at the "cheaper" end as well, offers lots of thrills with not many spills. That said as its not an actual track you don't get the feeling you have been on an actual racing circuit and therefore dampens the day a little... well it does me anyway.
Brands is a fantastic circuit, however it is actually quite technical, there are some tricky bends and elevations, not to say that you shouldn't give it a go, but if you do just go at a pace you feel comfortable with. There are some good run off area's however if you watch the BTCC you will know there are a number of areas where the barriers are much closer then you think.