1 - I have seen that marian reforms can be enabled in the BI descr_strat file. Do it work with BI ?
Yes & for the next inevitable question see discussion of how they are triggered here
(to make them actually do anything you'll have to add the marian reform requirement to units in export_descr_buildings.txt)

2 - Which file describes when and where do the Romano British appear ?
It is hardcoded but you can tweak it see here

3 - Never finished a BI campaign... When the Romano British appear, are they in horde (like the Slavs) or in a city ?
They come on looking like a horde but are not technically the same - see above link.

4 - Can we set a BC date for the beginning of the BI campaign ?
Probably, just try it you only need to alter the one line in descr_strat.txt eg:
start_date 363 summer to
start_date -363 summer
or whatever.

Oh and welcome to the forums