Further to
http://www.theanswerbank.co.uk/How-it-Works/Question1450814.html I thought, since there was a bit of interest in the issue, I would pass on what I have found out since posting the question.
It turns out that Royal Mail International Tracked & Signed includes more and better service than the more expensive Royal Mail International Tracked - that's official from Royal Mail themselves. They tell me that Tracked & Signed involves more detailed tracking info than does Tracked. Additionally, of course, Tracked & Signed ensures signature on delivery whereas there is no such provision included with Tracked.
Additionally, I enquired as to how a foreign Registered item is handled once it arrives in the UK and was at first told it is handled the same as inland mail. When I asked if that was first or second class I was told customs clearance varies so wildly it was impossible to say how long it would take for delivery. This was where I gave up, although I thought for example that there is no customs check on letters from within the EU or at least that non-EU/EEA is so radically different that it is worth mentioning this distinction.
The point I have come away with is that Royal Mail don't want to be quoted apart from that if an item hasn't been delivered within 15 working days in which case they regard it as lost, otherwise no "normal" and certainly no guarantee on anything.
My interest arose when I was expecting a foreign Registered letter from within the EU to which I would reply by some form of trackable international mail. The incoming was trackable from the location of origin abroad and online I was able to follow it into the UK - to HWDC, Langley. That is where the tracking facility ended and, while the letter was delivered last Thursday morning, as of now (Monday, mid morning) there is no further info in Royal Mail's tracking file - the foreign one depends on Royal Mail providing the detail so of course it too stops at HWDC. I find this distinctly unimpressive.