Yup, have to think they have some special counter-assault teams stationed around London, since ordinary training and arms would be unlikely to provide police who could quickly shoot 3 perps dead and (apparently) no “collateral damage” (at least I haven’t heard of any of the civilians with gunshot wounds).
One article referred to “police marksmen” shooting the terrorists, so that may be an allusion to specially trained up teams.
All the armed police in Britain are ‘specially trained’. Armed response units are constantly on standby in all British police forces, not just for anti-terrorism purposes.