Hi Sparrow,
I don't think you necessarily have to have a relationship with Brighid to celebrate Imbolc. Personally I have never worked with her, but I love this time of year because it signifies for me the returning of the light and the end of winter drawing close.
I agree with Jess. Imbolc can be seen as the celebration of the first glimpses of Spring, Winter coming slowly to an end, the first flowers are coming out. IMHO not all Sabbats are necessarily linked to Gods or Wicca but to celebrate different times of the year like the first Harvest, the beginning of Winter, the glory of summer and sun.