These are all huge universities in a huge city, the nightlife is honestly what you make of it. There are plenty of student events and club nights that are always happening so you will not have any shortage of those, especially in freshers' week.
With regards to the social life, it depends on how much effort you put into meeting new people and developing/maintaining those friendships and connections. As I mentioned, they are huge universities which means that you have a lot of people you could meet and interact with but the other side of that is it's easy to be very anonymous. They are not campus universities as such since they're very spread out and integrated into the city and so you do have to try a little harder to maintain social circles compared to say Oxbridge.
Personally, the nightlife and social life at UCL were great; I would very much recommend joining clubs and societies as these will be the best way to meet and bond with people.
Also, I'm not really sure what you mean about UCL/LSE not being great because a 'lot of students are foreigners' - are you actively avoiding foreigners? If you're concerned about language barriers, there are thousands of international students who speak perfect English so I wouldn't be worried about that.