You should bring a wetsuit to every event. Under BTF 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 15.9°C, Mandatory below 15.9°C (up to and including age 59)
Up to 1500m: Forbidden above 22°C, Optional between 22°C and 15.9°C, Mandatory below 15.9°C (age 60+)
1501m and above: Forbidden above 24.6°C, Optional between 24.6°C and 15.9°C, Mandatory below 15.9°C (all ages)
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 allowed.