PhD prospects in law with a low 2:1

I have a first in my law undergrad and llm from my local institution (outside uk, not great ranking) and a second masters from a top UK university - with a low 2:1. Will the grades in my second masters work against me if I want to pursue PhD in one of the top UK universities? I plan on applying for a PhD in 2/3 years. What can i do in the meantime to increase my chances of acceptance/funding?
P.S. The area in which I want to do a phd is not strictly relevant to my masters courses

You will be less competitive than other applicants that have a higher grade, but as long as you meet the minimal criteria you can apply and fight for your place.
Each university have different requirements - for example, PhD entry criteria for Coventry University states:
"A minimum of a 2:1 first degree in a relevant discipline/subject area with a minimum 60% mark in the project element or equivalent with a minimum 60% overall module average"
Publishing Linkedin articles (unofficial/unpublished research) or scientific research (published reviews, etc.) into the field you wish to enter would be a great place to start. Research work experience would also be demonstrate your interest in academia. You could also do online or face to face courses related to your PhD to demonstrate your interest and expand your knowledge in the field you wish to enter.
I say you try and see how you get on?

If you are looking for funded places, the prospects are not great...if you are self-funded, however, you should be fine, especially being an international student. I know someone with a low/mid merit getting offers from both UCL and Durham.
Good luck on your PhD quest!

Thanks for this! Do you mind sharing whether publications would help - is the person you mention someone who has work experience in academia and/or publications in the relevant field?

Publications in peer-reviewed journals will definitely help. Not sure about other media. However, I do think that you are overthinking things. I would say that with the possible exceptions of Oxbridge, or at a stretch, maybe the LSE, you should be OK at most other UK universities. That is, if they have the right people with the expertise in your field to supervise your research.
Make sure you have a fairly decent research proposal, or at least, a good idea of your focus of research, before you contact potential supervisors, as their interest will help with the application. Some places, like Durham or Glasgow, do not encourage you to contact potential supervisors, but instead, require you to make an application first. I would always advise that you are happy with the supervisors before accepting any offers, though. They will be the most important people in your life (your academic life, anyway) for the next 3 to 4 years.
The person I know that got into UCL and an offer from Durham with a low/mid merit had no previous research or industrial experiences, and no publications.

I have a first in my law undergrad and llm from my local institution (outside uk, not great ranking) and a second masters from a top UK university - with a low 2:1. Will the grades in my second masters work against me if I want to pursue PhD in one of the top UK universities? I plan on applying for a PhD in 2/3 years. What can i do in the meantime to increase my chances of acceptance/funding?
P.S. The area in which I want to do a phd is not strictly relevant to my masters courses

To be honest, I don't think 2/3 years is enough time to make you competitive for funding, unless you are doing something epic in that time which forms the foundation of a fascinating research proposal. So if you are an assistant on a government enquiry form which your research proposal follows on, and you have all the contacts etc, then possibly.
PhD offers are relatively easy to get (harder than Masters, but there's still an element of over-offering), but funding is much harder, and full funding very difficult indeed. Law will tend to have endowed funding, rather than be commercially funded (like the sciences) and so is even more competitive. You will need either an epic research proposal, or more time to build relevant career experience.