It's about a lot more than the total weight (although weight distribution matters).
Tyres are far and away the most important thing. If you are on the wrong tyres, or if you are in the right tyres but they are worn out, you won't get any grip. Ask at the track for some tyre advice, the local drivers should know what works best (probably Schumacher Yellow Minipins or Minispikes).
The second most important thing is the car build. You need to make sure it is mechanically perfect, every moving part should be free running, every fixed part should be solid. A binding shock or a locked bearing (for example) will do more damage to your car's handling than any setup change.
A B4, built well out of the box, with a good set of tyres and a sensible motor should be driveable in most UK conditions with just a change of springs (silver all-round is common).
To directly answer your question, I always tend to run my car light (close to the BRCA limit of 1488gm), with roughly 66/34 weight distribution (which is more or less what a B4 builds to anyway), but other people choose to ballast the car up to higher weights by filling the car with as much brass and lead as they can find. It's a tuning choice.
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