A 2.5mm cable is capable of supplying around 23-25 amps depending on the method of installation, so is fine, safe and compliant on a 20amp circuit breaker.
2.5 mm is most commonly used for ring mains, plug and socket wiring. 4 mm is most commonly used for high power ring mains, plug and socket wiring. 6, 10 and 16 mm are most commonly used for high powered small appliances such as cookers.
6327G6 ANDERSON SB175 max.175A wire AWG4, electrical mount crimped