What do you mean by enabling the Romano British? Do you mean make them playable? If so, go to data/world/maps/campaign/imperial_campaign and (backup first!) open descr_strat.
Ctrl-f to Romano British, and when you find them, you should find that area, delete certain lines (I forget the specific lines and I no longer have BI installed), I think "dead_until_resurrected" is the main one you need to delete. Copy any army from any faction into the Romano-British army section, and replace the units with Romano-British units (the names of which you can find in export_descr_units in data). Give the new army coordinates similiar to the Londinium army, (increase or decrease each variable by one or two). I forget the name of that army's leader, just boot up a game as the WRE to find it, then ctrl+f the name in descr_strat.
Next, in the data folder, go to descr_sm_factions (something like that, if not that, ctrl+f faction), and make the Romano-British a nomadic faction. This is important for allowing them to start with no provinces.
If you want them to start with Londinium, skip the descr_strat step, and move both the Londinium army and Londinium itself to the Romano-British section in descr_strat.
For the historical battles, go to world/battles/battle_name, open descr_battle, and move the factions in "nonplayable" to "playable.
If you want to play as most of the emergent factions, I made a mod for it ages ago:
http://www.strategyinformer.com/pc/r...mod/37247.html