doing masters while working? I work full time as a bit of a mix: sql server DBA, BI developer, DW admin, ETL developer…

I hold a BS in Systems Engineering, and wanted to continue studying…

What are my options to balance a possible Masters in business intelligence and at the same time working full time?

I live and work in SouthWest London. Work is a bit flexible (I would be able to modify my working hours a little bit).

I have no knowledge in current options, but; is there any university in London where I would be able to do a masters in BI, attending to classes maybe only Friday and Saturdays?!
I am currently doing the exact same thing you are wanting to do: working full time as a data engineer and pursuing a Master's degree part-time in data analytics. It took me a very long time to find a program that was focused on BI or anything near that since it is still a relatively new field (new as in gaining traction in academia). Also, Business Intelligence is very wide, it encompasses a lot of different things. University College London (UCL) offers a part-time master's degree in Data Science which encompasses some BI stuff and more. If that's not exactly what you meant by "BI", then I would recommend searching for schools in London (or worldwide, in which case you'd be looking for online programs - which is also very possible) that offer a BI program, and then seeing if it can be part-time or not.