Premeditated yes, but seemingly less so than most recent jihadi attacks. Hiring a van and driving it to a particular mosque in London doesn't require that much planning, after all.Renting a truck in Cardiff and driving 100 miles to London to use it for an attack on a mosque sounds quite "premeditated" to me.
You can also judge the severity of the attack by the number of casualties, which again is lower than most jihadi attacks we hear about.