Ta is used in place of thanks. The Urban Dictionary says that its roots come from Danish. Thanks in Danish is tak and in England and Scotland they dropped the k.

Jane and I have had much discussion about this. She says that she would never use ta especially in emails. I told her that I have heard it in the places I have worked and my sister-in-laws will say it (but they did live in London for a bit). I guess you are influenced but the people who are around you.

All the Best!

American to Britain