I have found out that I have a blood group that is one of the rarer ones in the U.K. where I live. It is B+. I understand that Jewish people are mostly B+ also. My birth surname is Low which I gather is a Jewish name. Would I be correct in assuming that I am of Jewish decent?
This is not enough data to suggest that you are of Jewish decent. It is true that B+ blood type is more common in Israel (17% of the population) than in the UK (8% of the population), but it is much more common in India, for example (31% of the population) or in Estonia (20% of the population), where there are not many Jewish people.
Originally, Low was not a Jewish surname. Unlike "classic" Jewish surnames, it has no meaning in Hebrew. The name Löw, usually pronounced "laoo" (at least in Israel) is a relatively common surname of Jewish people, but it is of German origin.
To find out if you are of Jewish decent you should check genealogical data. Anyway, you are considered Jewish only if you mother is Jewish.