ACPO has commissioned the FSS to cary out definitive testing for lethality and the results are out.
ACPO now consider 370fps full auto and 520fps single shot as the levels where lethality kick in (1.5j and 2.5j respectively) Anything over that is a firearm (air rifle) so full auto guns, are considered section 5 firearms over that.
Single shot is anything that discharges a single shot with one pull of the trigger, so includes semi auto.
We used to operate in a very convenient grey area regarding lethality and had a lot of leeway as nothing was certain, that leeway has now disappeared.