Barbel will feed and will feed well throughout the winter but it very much depends on the temperature.
The best conditions are during a warm flood but the feeding window will be limited.
Unless you have local knowledge and are able to fish at short notice then the odds are against you.
They will feed in cold weather but only if temperatures are stable and you are able to fish in the dark in a place where you know there are barbel.
Unless you know the river well I would not even think about fishing in a flood. You really do not want to be on the banks of rivers like the Severn, after dark, in rising water....it is very dangerous and finding barbel would be like looking for a needle in a haystack.
They are very pedictable and very catchable in the right conditions, at the right time and if you know they are likely to be in your swim.
However the winter is not a good time to try and catch barbel unless you are very flexible and can fish at short notice and know the likely holding areas.
They will certainly not feed in cold conditions like chub.
The popularity of winter barbel is because they tend to be at their heaviest in the winter but bear in mind that many anglers stop fishing for them in November because of the difficulties mentioned.