LSE history is not "largely economic history" (that's a separate course and department) and the economics optional modules still expect you to have the same background as the economics students (because they're the same modules...). Just pointing out for posterity.
If you're keen on studying Russian (or any East European language) then it probably really is worth strongly considering SSEES. They are specialists in the region and the languages after all. While LSE does teach Russian they aren't specialists in that (they don't have a specific department for it and likely don't have the same depth of pedagogical experience in delivering it), so you may find that you aren't going to attain as high levels of proficiency in the language and/or it may be taught worse.
That said, bear in mind that the economics modules are taught by SSEES, not UCL Economics. Their modules are somewhat less mathematical (it does not appear the first year modules require A-level Maths, and there is a module which is compulsory for first year students in HPE without maths in mathematical methods to give you a similar background). While this is neither here nor there for your purposes for an undergraduate degree, it's probably worth bearing in mind this is unlikely to be something that will set you up to e.g. pursue a PhD in economics or similar.
Your primary interest seems to actually be economics, in which case I'd have suggested just taking a gap year and doing A-level Maths and ideally Further Maths (at least to AS level). Ultimately economics is a necessarily mathematical subject outside of the artificial limitations of the A-level syllabus, and if you aren't mathematical capable and willing to do maths extensively in a degree, economics is simply not a good choice.
You might want to spend some time reflecting on why you seem so fixated with economics, in spite of not apparently wanting to do and not having done the kind of things economics students do (i.e. maths).