You should bring a wetsuit to every event. Under British Triathlon rules wetsuits are either mandatory, optional or forbidden depending on the combination of swim distance and water temperature.
Up to 1500m: Forbidden above 22°C, Optional between 22°C and 14°C, Mandatory below 14°C
1501-3000m: Forbidden above 23°C, Optional between 23°C and 15°C, Mandatory below 15°C
3001-4000m: Forbidden above 24°C, Optional between 24°C and 16°C, Mandatory below 16°C
Mandatory wetsuits must cover at least the torso.
You can wear any type of wetsuit including shorties.
Where wetsuits are mandatory, socks are permitted. Gloves are not permitted.