Economics as an A Level isn’t even needed when applying to Bsc Econ at LSE. They give offers to quite a few applicants each year who haven’t taken Economics A Level - simply because they teach the Economics from scratch from what I’ve heard, whereas the maths knowledge is a prerequisite and is required.
Mentioning that your school didn’t offer Economics in your reference isn’t as necessary as mentioning Further Maths, but you may if you wish to do so.
And to answer your question, of course not. Most applicants wouldn’t have taken the TMUA, and only those who’ve applied to Cambridge. Some people may be applying to Oxford as well and have taken the TSA instead, which the LSE don’t consider. I’ve heard that LSE don’t look at the TMUA anyways for pure Econ, with people with a 3.0 score still getting offered a place. In a sense I would assume, no TMUA is probably better than a low score in the TMUA and then not sending it to LSE even though you said you’ve taken it. There’s also the rumour that LSE tend to reject Oxbridge applicants to maintain their offers/acceptances yield, which I doubt is true but considering how crazy LSE’s admissions processes are, it’s not too far off to think it could be something like that.
Although the TMUA is a bit more desired for an application to the Bsc Maths and Econ joint honours course at LSE.