My understanding is that it's both. If you bring up an alibi during trial, but never mentioned it during interrogation, it is an indication of guilt. If you don't say anything at all during either trial, or interrogation, it is also an indication of guilt.

I'm not sure how much difference it makes in practice. It's the jury that decide. A jury in the US might equally be suspicious of an alibi that the defendant didn't mention until late in the process.

Also, it's not really true that merely remaining silent is taken to be an indication of guilt. The point is that an adverse inference may be drawn from silence in basically the same way that an adverse inference may be drawn from a statement made to police. The prosecution can't just say "the defendant said nothing when questioned and is therefore guilty".

The jury can, of course, decide to convict a ham sandwich, or to let a blood-on-his-hands killer go, but the instructions given to the jury differ. That difference matters. If it didn't, the US could safely do away with the fifth amendment, and nothing of value would be lost.[1]

The point is that in the US, the prosecutor can't even suggest (well, they can suggest, but an objection will be raised) that the defendant's silence is an indication of guilt.

[1] Anyone who was compelled to testify against themselves would just point out that they did so under duress, and surely, a reasonable jury would throw that testimony out. I certainly would. For some reason though, we don't expect juries to be reasonable on this point.

I'm not seeing the analogy with the Fifth Amendment. The point about the Fifth Amendment is that you can't be punished for refusing to offer self-incriminating testimony (e.g. by being held in contempt of court). So it's not merely protecting you from whatever prejudices the jury might have about someone who refuses to testify. It's preventing you from being punished for the mere act of refusing to testify (whereas you cannot punish someone in the UK merely for refusing to answer police questions – except in a handful of very specific circumstances).

Indeed, yes, prosecutors have more free rein in the UK to suggest to the jury that someone's silence during question is incriminating. I just wonder if this makes much of a difference in practice. Do you know of any examples of cases in Britain where a conviction would most likely not have been obtained if the prosecutors were not permitted to do this?