All the Oade SLA packs have the AH rating in the name. BP3.2 = 3.2 AH, BP7.2 = 7.2 AH. Soooo...sounds like you have an older pack if it's a BP2.8, and it likely = 2.8 AH.
Your UA5 draws about 450mA. I imagine you'd get somewhere around 4 hours with it. If you're not in a hurry, best bet is to test it out at home.