This week the results of lethality tests performed by the Forensic Science Service were published by ACPO (Association of Chief Police Officers).
The full report can be found here ACPO cover letter and here FSS airsoft lethality0001 (accompanies letter) however the relevant information is as follows:

ACPO Firearms and Explosives Licensing Working Group (FELWG) have proposed that the following power levels should be regarded as the lethal threshold for airsoft BBs (i.e. the power level at which they cease to be regarded as replicas and then become, on paper, real firearms). Nothing has changed in law but most police forces have adopted this as policy, so will very probably prosecute any skirmisher who exceeds them. In simple terms, airsoft skirmishers should now regard these figures as the maximum UK limits for possession, sale, purchase, import and use.

The document has established the following recommended threshold of lethality for airsoft BBs:

Full-auto or burst-fire capable : 1.3 joules (around 370fps with .2g bb)
Single-shot only (including semi auto) : 2.5 joules (around 520 fps with .2g bb)

This means that a full-auto airsoft gun over this limit is now deemed to be in the most restricted category of firearm, section 5 firearms, is only available through a Registered Firearms Dealer (RFD) and would not be useable in airsoft. The minimum penalty for possessing a section 5 firearm without a section 5 licence is 5 years.

A single shot or semi automatic airsoft gun over 2.5 joules is now deemed an air gun and can be sold to anyone over 18 (as it is not subject to sections 36 to 41 of the VCRA 2006), but is probably illegal to fire at another human being and therefore also unable to be used at an airsoft site. Air guns are also subject to a number of other laws.

All types of airsoft replicas are affected by this including spring, gas and electric powered guns.

We are awaiting response from PSNI to find out if they will be adopting this recommendation in preference to the current 1J policy in Northern Ireland. There are still other areas of this subject which are unclear, such as the status of gas automatic replicas (the muzzle energy of which can vary greatly), whether it will be deemed acceptable to downgrade a post lethal AEG and whether a semi auto only 1.3-2.5J AEG or even a sub lethal AEG will ever be regarded as ‘readily convertible’ (and thereby section 5). Again, we will forward any information we receive.

These results do not dictate an increase in UK site limits as site limits can of course be set by the game organiser at any level below the recommendation. Fortunately the vast majority of UK skirmishes are already played within the reccomended limits. UK airsoft is in a much better position now that these results are out as we now have a much better idea of how airsoft replicas will be regarded.

UKAPU recommends that 1.3J/2.5J is adopted by all UK players as the maximum limit for airsoft skirmishing replicas, with immediate effect.