If you're a big fan of history like I am then you'll probably enjoy programs such as Vikings or the last Kingdom.
These excellent productions are brilliant in dialogue design and overall great storytelling. Unfortunately Britannia has none of these qualities except maybe In design. The show itself is slow paced, with cringeworthy dialogue and overall all I found it very boring. There's much better historical shows out there are and I would seriously give this one a miss.
OK here is the rub you have a guy who can only be killed a certain way and the world is on the brink of annihilation in the future, what do you do? Well you recruit the worst super heroes of all time that's what. For some reason these guys of mixed super hero ability can not seem just kill one guy and the series is drawn out with incompetent action sequences and terrible story lines. This whole show should have been over in like 3 episodes, but somehow they seem to use every time line to basically mess things up and make matters worse.
The effects are probably the best part of the show with some nifty CGI and sets. There really needs to be more bad characters as everything is focused on one guy and the heroes don't seem to be half bothered about getting the job done as they have so many issues themselves.
Overall the show is watchable just don't expect Flash or Arrow watchable.