Right now, you will be accepted almost anywhere you apply for a Masters in the UK, so I would definitely suggest looking at the same university level or higher - do not limit yourself. Your first degree is quite relevant to Statistics, which gives you an edge, and besides already holding a Masters shows your level. I am confident you will not have a problem being accepted even in elite institutions.
On the second part of the question, on Sociology or Political Science, I am not clear on what you have in mind so I can only suppose you intend to apply for a PhD. There are many areas of both that use statistical analysis (e.g. voting patterns) and a quantitative background is always an asset, and Economics is always relevant to Politics. It is definitely relevant but puts you in the front row only for some areas - a more traditional choice like International Relations or History and Philosophy might be closer to a "one size fits all" approach to PhD applications in Sociology and Political Sciences. However, a candidate with your suggested background will be highly sought after in specific areas to the forefront, and there is a movement of Political Science research towards quantitative methods, so your second degree will be a very good fit.
Overall, it boils down to what interests you and I have no clue on that, but EconFin + Stats is quite a competitive background, especially for the private sector, and highly sought for a wide selection of Social Sciences research.