I already have a small airline flying mostly into northern Europe from luton. Would you suggest staying with this or going long haul?
there is no real limit of what you want to do, as long as you don't commit 2 aircraft of a new type and back out. Always do your research to see if there is demand for the routes that you want to fly before opening a new base/new type of aircraft.
One thing to note is make sure you leave some money if you want to try longhaul(flying to NA). It takes quite some time for the route to build image and until then you may lose money on the route.
+1 : the longer the route, the more it will hurt before bringing profits. The safest way to begin is short lines with a lot of demand.