Math, Chemistry, Biology and Physics.

You better have Chemistry and Biology, both to A level, plus Maths and Physics if you can. The minimum requirements are often not very demanding, and expect fellow candidates to have more than satisfied those.

Taking more subjects does do good (assuming you do well in them) because in the UK and elsewhere many students take more, and places are limited as it is.

Someone who took 5+ subjects among Maths, Further Maths, Economics, Physics, Chemistry, Biology, and English Literature would have a far greater outlook (in terms of university applications for every kind of course except strictly non-scientific humanities/arts/languages subjects like design, French, history, but not including social sciences like politics, government, anthropology, and satisfying course prerequisites of course).
These four are very difficult, assuming they're done within 1.4 years. However, getting As are not very difficult (you will rarely be asked for more than 1 or 2 A*s at even the most competitive of schools).

What do you mean by P1 and P2?

Sciences have P1 to P5 (AS = 1, 2, 3 and A2 = 4. 5) while Maths has options (2 AS and 2 A2 with conditions from P1 through 7).

You can apply anytime, it is only A level after all.

1.4 years was the time it took my class (and others from the same intake) to do it. Like I said, there're 2 intakes so timing is different.

For the best of the best, and even some below those, 5A*s is never enough for an extremely high chance of getting in. Even assuming your predicted grades are 5A*s (and you actually achieve these after your application), it is ultra competitive, to the point where someone with lower grades (maybe fewer A*s) but a better application and history overall could be better off. Some schools ignore predicted grades (or dismiss them partially), and needless to say AS grades are relatively identical for most applicants so it is not given thought (when deciding amongst qualified candidates). Luck is far more important than one might think.

Yes, medicine would be good with those four. There are alernatives, but almost none are more appropriate.