Well, that would be down to the law of the land. An airsoft gun that is "capable of discharging a missile (of any kind) with kinetic energy at the muzzle of the weapon that exceeds the permitted level" [of] "in the case of a weapon which is capable of discharging two or more missiles successively without repeated pressure on the trigger, 1.3 joules;"
http://www.legislation.gov.uk/ukpga/2017/3/section/125/enacted
1.3J is about 375 fps with a 0.2g BB. If it's capable of producing more than that, it's a section 5 firearm.
Since you're so sure of yourself, you could get your local firearms unit round to check out out. Have a good old angry-man at them about how some spods on their interwebphones are talking jive while you rip off a 1.63J full auto burst.
But being sold as full auto. We all know what you mean, but given that the wording of the statute is "capable of", and since you're much bigly smarterer than us, you might want to consider listing it as being locked to semi-auto.
This is for your benefit, and that of any potential victim who buys that from you, not ours.