I assume you are asking this as a rhetorical question, but I'll answer anyway to vent.
As we know, the 1994 AWB was sloppy since they were not really banning true assault weapons and would then have a clear definition, but some made up collection of features they called assault weapon, such as a bayonet or attachment point, flash hider, new 30 rd mag, etc.
So manufactures removed the bayonet lug and flash hider, installed a muzzle brake and called it a Sporter. Models of collapsible stocks (how evil!!) were made into fixed ones to sell after the ban. AKs were further restricted if they had less than 10 parts made in the US, so US parts kits came out.10+ rd mags were grandfathered so all the old USGI mags in surplus came out of the wood work, plus there happened to be cargo containers full of AK mags. I seem to recall AK mags were under $5/ea, maybe $3/ea in bulk, and Ar mags around $10/ea or less before the ban. Huge surplus market.
Being CRIMINALS the NHS shooters did not care much for the laws about gun restrictions since they planned to rob a bank and lay down a wall of lead on anyone who tried to stop them.
As far as I know NO ONE has ever been able to show that gun bans of any sort reduce violent crime. And apparently in Britain, neither are effective bans on knives, swords, bats, hair pins, pens or pencils, or any other object used for self defense. ans only disarm the law-abiding and enale the criminal or tyrant to have the advantage.
Finally, from what I have read of the shootout, the police did in fact go to a local gun shop and get AR-15s but SWAT arrived with their own rifles before the civilian ones were deployed.
SWAT eventually arrived with rifles powerful enough to penetrate the body armor. Several officers also appropriated AR-15 rifles from a nearby firearms dealer. The incident sparked debate on the need for patrol officers to upgrade their capabilities in similar situations in the future.