Rain
Rainfall is quite variable at this time of year. Scotland, Wales and Northern Ireland usually have more rain than England.
Take care not to tread on or work the soil when it is really wet - you will likely do more harm than good.
Sunshine
This can be one of the worst months for sunshine levels, approaching the winter solstice. The sun is also much lower in the sky at this time of year, which means that it hits glasshouses at such an angle as to make its transmission and usefulness less than in the spring or summer months.
Visibility can be further reduced by hill fogs and fogs over inland, sheltered areas. These will take longer to disperse in the cold, damp, gloomy weather at this time of year.
Light levels can limit vegetative growth of plants just as much as low temperatures. Giving extra heat to greenhouse plants will not compensate for this lack of light, and your plants may become leggy. Some plants (e.g. poinsettias) respond to reduced light by induction of flowering, or the colouring up of bracts.
Temperature
Night frosts are to be expected in all areas except for the mildest coastal regions of southwest England and in the shelter and stored heat of cities.
Frosts will help to improve the crumb structure of previously dug soil, and may kill off some pests and diseases. Tender plants will suffer unless protected.
Wind
Gales, gustiness and cold winds are to be expected. This, combined with driving rain or snow, can make for very unpleasant conditions. Frosty days, by contrast, are more likely to be still and clear.
Staking, tying in and protection of plants is a must. The wind can be destabilising, physically damaging, chilling, and very drying for plants.