As for sticking out like a sore thumb ...a Harpoon or Exocet coming in at high-subsonic will stick out just as much as this British missile (and other supersonic, and soon to be hypersonic, missiles screaming in) when it comes to viable detection on a radar system. Both will be very easily detected and tracked (now, if the Exocet or Harpoon was inching forward at 2 feet an hour you may have a point, but that is not the case ...thus if the Brit missile is sticking out 'like a sore thumb' then the Harpoon/Exocet would be sticking out 'like a sore index finger').
The same applies to IR detection ...sure, the frontal cone temperature of an approaching multiple-Mach projectile will be significantly higher than that of a high-subsonic projectile, but both in terms of IR radiation will be high enough to be detected by any (US or foreign) IR systems. Once again, using the 'sore finger' hypothetical-subjective scale, if one is a sore thumb, the other in this case would be a sore little finger (at least not index finger like the radar scenario) ...but unfortunately very clearly detectable.
Long and short is that either missile, supersonic (or even hypersonic) vs high sub-sonic, is very easily detectable to the 3 main types of detection: i) radio-frequency, ii) electrical-optical and iii) infrared visibility. Very easily. Which is why future missiles are taking two approaches ...the first is sub-sonic stealthy missiles (primarily RCS and IR reduction), while the second approach is speed (supersonic and hypersonic approach). Some people like the Norwegians have taken the stealthy route for their next-gen ASM (in particular, the stealthy NSM). Others have gone for the other camp, and these include the Russians, the Chinese, the Indians, the Taiwanese, and now the British (article above), and the United States are working on the super/hypersonic angle. Yes, the US is working on the LRASM-B, which is a super-sonic ramjet powered missile to be used in the vertical launcher of ships. Interestingly, that is what they decided to use on ships, rather than LRASM-A which was a sub-sonic stealthy variant that they opted not to include on ship-strike but rather as an aircraft carried weapon. Pic of LRASM-B attached below. It seems that there is a much larger consensus about the survivability of a supersonic (and hypersonic) incoming projectile. Again, it does not make the weapons invincible by any means (and future anti-missile technology will get better, e.g. work on laser based defense systems), but they are FAR more survivable than sub-sonic missiles. 3 seconds versus over a hundred seconds is a lot of difference, and future ASM work and investment seems to be cognizant of that difference.
