Just so you know: The whole Roman era founding is a lie, FYI. You could easily conclude this by looking up Caesar's historical incursion's into Britannia and discovering that the one CG says was a failure beaten back by unified Celts was historically a failure for several reason, the least of which were it's inhabitants...and it was followed up by a second invasion that was much more successful and yet Britannia was not actually "conquered" until something like the ~200s AD. More importantly, such a thing would have led to enormously vast changes throughout the two thousand odd years since then, like Napoleon never being born kind of vast.
There's also this but I don't really know where it came from:
http://i22.photobucket.com/albums/b3...2253778363.jpg
As for the ATB timeline, from all appearances it's a lie (Or a very massive oversight) as well. The dates for things like the early 1900s and Napoleonic events are not 50 years off from when they historically happened. Furthermore, most of the technology displayed in the year 2017 ATB (Lets just leave R2 out of this...) is not at all unreasonable to expect in our year 2017 AD aside from the vastly superior batteries courtesy of a material that does not exist in our world.
tl;dr - All of Britannia's pre-Tudor era history is probably a lie. I think it's a pretty simple explanation and well, Occam's Razor.