What would be the best single threaded LP solving algorithm that can used in Xpress. Or how can i limit my problem to solve using only one thread

The DEFAULTALG that Daniel mentioned selects the LP solver for every LP during a MIP solution process except the initial root LP. For the initial root LP, consider using LPFLAGS instead. Note that LPFLAGS is a bit flag control. For a more detailed explanation, see also my answer to this community question.