Cambridgeshire conducted an exercise on a Saturday morning during last year (they didn't inform anybody it was going to take place only officer types) to Mass decon incident - Make IRU's 10 'decontaminate 2000+ people in London'. We all (10 pumping appliances, IRU, mechanic, PCV and some officers) blue lighted it to Brampton services aka the rendevous point. We then were told we were going to South Mims services so on with the blues again and off we toddled. We got another 10 mins down the road and were turned back.
All pumps mobilised were from Cambridgeshire. Cambridgeshire Ffs were not told it was an exercise - we summise from this that it was to see if everybody that was mobilised actually turned out. As it happened everybody did!
I take it from our training input that each brigade that has an IRU when mobilised form there own column (10 pumps plus support vehicles) at a pre determined rendevous point.
Unfortunately with most shire brigades forming a column will seriously deplete the fire cover for a considerable distance within the brigade until standby pumps are moved in.