In Britania campaign nobody can blitz thx to ridiculou amount of insurgents that wreck your settlement if you decide to leave it. Leaving 3 units of whatever won't work, since a governor is the ONLY method of keeping a place in check and converting culture. A church does do the job but a lot slower, and if u wait for churches to be built then ure not blitzing.

If you would blitz in Britannia you would probably have to exterminate any settlement above large town and move on while norwegian raiders will (they only hav a chance to survive by raiding and abandoning settlement) chew up your undefended backwater.