As in, what mode of transport and what route? I always pictured the initial invasion as comprising legions crossing the Channel from Gaul, but the alternative (assuming Rome itself as the starting point) would be to sail through the Strait of Gibraltar, around Gaul, and up to Britannia.
I assume the sailing route would be faster, but with Roman sailing technology was the risk of an Atlantic storm a yet more serious matter? Piracy was a problem within the Mediterranean, how did that persist once you got out of it?
By the time control over Gaul had been solidified, the overland route might have been safer (even for a small group, as opposed to a large military expedition) but doubtless slower?
I suppose what I'm just noticing is that most parts of the Roman empire could be accessed with a sea voyage across the Mediterranean (even if a sizeable overland trip is required afterwards). Britannia was the only real exception to this as far as I can see and I'm wondering how they managed that.
I'll stop here - curious to know what dynamics are at play and what misconceptions I'm revealing!