Quite Strange SIS and MI5 did not caught this one in time to be honest.
From the last attack (The Ariana Grande Concert attacks) is merely 11 days, the person responsible is most likely the same group of people and preliminary intel suggest these group is also on SIS watch list, but still, both GCHQ, SIS and to some extend MI5 did not do anything for it.
This attack is also called "Ripple Attack", this cell is most likely have some sort of connection and/or relation toward the May 23 Attack, and the effect of the last attack was brought on with this one.
However, there are also chances that this is an individual attacks (ie two or more cell operate in the UK) but somehow I feel that this attack was let down by the British Intelligence Community