I need a BB and distinction to do social science at Royal holloway. however I might get a BB and a merit... are Royal holloway lenient with grades?
No university is "lenient" with grades. However, some universities / courses are more likely to have enough candidates who actually achieve their offer grades, that they don't need to accept candidates who've missed their grade requirement. Others are not.
According to The Uni Guide, the most popular A levels studied (and grades achieved) by Social Science students at Royal Holloway as Sociology (B), Psychology (B) and Economics (C) - source. The fact that these aren't BBB suggests that in the past, Royal Holloway have accepted some "near miss" candidates (as you would be).
They doesn't necessarily mean that this will this year - it simply depends on how many offer-holders meet their offer grades.