Hi there, I took the tmua last year and received an offer for MORSE from Warwick.
Firstly, if you want advice as to when you should start, it goes without a doubt the earlier you start, the better. R2Drew2 is an excellent resource for the TMUA, would greatly recommend all of his videos, he goes into tips for the TMUA, and has hundreds of hours on the TMUA, watching all the explanation videos would be a must from me, especially if you are initially struggling.
Secondly, you should not be too worried, I started revision 2-3 days before, and I was not particularly strong at maths at the time as I was not focused on a maths related degree and was more focused on economics, taking it mostly for Cambridge. I kinda flopped it and got a 4.3 but it was enough for a Cambridge interview and a morse offer, so regardless if the score is quite low, anything above a 4 would probably be enough for morse. MORSE gives out loads of offers, I think it was about a 80% offer rate or something on the same line, just attached with high requirements. If you get 5.0 or high you get a reduced offer, but as I didn't get that, they simply gave me an offer of A*A*A*A (A*A in maths and FM), or A*A*A* (With A*A* in maths and FM)
for applying to morse, as long as you take further maths and have high predicted, you should be fine as long as you do okay in the TMUA.
so basically start revising as soon as possible but don't worry yourself too much, especially if you are applying Cambridge or LSE, don't leave it last minute like me and good luck!