How can I get the $delta of block created using UI?
If you go to admin/structure/block and mouseover the "configure" link you'll see something like this for blocks added through UI:

/admin/structure/block/manage/block/1/configure

something like this for blocks created with views: 

/admin/structure/block/manage/views/81e6b321879efa58ba9e546521c14467/configure

something like this for blocks created with webform: 

/admin/structure/block/manage/webform/client-block-50/configure

something like this for blocks created by the system (core):

/admin/structure/block/manage/system/main/configure

I've bolded the deltas and italicized the modules in each example.