Hi,
Taking Arabic floating units is possible; on a single honours law BA you can do 1 course (25% of the year's work) per year after the first year. The Arabic BA and any joint honours degree (Law plus Arabic for example) will involve taking A LOT of Arabic, its 75% of the work load (3 units) in the first year, one in 2nd, a whole year abroad only for Arabic and 1 in 4th, plus other literature units. By contrast, taking it as floating units means you start off taking the huge course designed for first years (3 units) but broken up into three parts, one per year, so its the same amount of work but will fit with the rest of your degree. This is called Arabic 100, 200 and 300. If you somehow get past Arabic 300 level you can start on the 1 unit courses normally offered to Arabic BA students in the 2nd, 3rd (for those not abroad) and 4th years, they are called Arabic 2, 3 and 4. Most people with no previous knowledge of Arabic will only get to take 100 and 200 though, which is a very, very basic level to end up with, possibly not worth having.
I would ask yourself why you want the Arabic and whether you are willing to apply for the Law/Arabic joint honours, that way you would have a very good grounding in it. Or whether you want to take Arabic 100 + 200 courses plus spend all your summers studying Arabic abroad, that would mean you might get to the end of Arabic 2 by the end of your degree, that would be a good level if you were prepared to continue with Arabic after graduating.