I have been accepted into Southampton online IT LLM and Strathclyde online LLM Internet Law. I plan to proceed to PhD for law teaching. Which is the better option?

Depends on the area you want to specialise in and whether there is a recognised (published) specialist in that area in either university.

I think doing the masters online might limit your opportunities to get to know the professors which is really important if you plan to study under them for you PHD or avail of their network of contacts in looking for a PHD position elsewhere.

Congrats on being accepted!