kaitak, the regiment which defended Rorke's Drift was not the Welsh Guards; it was in fact 'the 24th. Regiment of Foot.' A couple of years later the British stopped just using numbers and began giving regiments names as well; and the 24th. became 'the South Wales Borderers.'
There is a connection with the Welsh Guards, though. Here's a version of the 'proper' words of 'Men of Harlech' - which, as far as I know, is their regimental march. It's being played as they take part in the annual 'Trooping of the Colour' ceremony in London.
Just going by its (low) number alone, though, the 24th. was quite a distinguished regiment. It was indeed first raised very early on, in 1689. It's full 'parentage' is shown here:-
As you can see, it was aptly named the 'Borderers,' because it traditionally recruited both Welsh and English people; and was even, at one stage, called the '2nd. Warwickshire Regiment.' The regional names of British Army regiments weren't always overly logical - my own father, called up at 17 in 1917, found himself in a mob called 'First Battalion, 18th. London Regiment, 'London Irish Rifles'; even though neither he nor any of his mates had any connections at all with Ireland! He put that right later though, by surviving WW1 and marrying an Irish girl!
I'm not sure that the Borderers would have sung 'Men of Harlech,' though. For a start, Harlech is in North Wales, not South Wales; and, for good measure, the song itself commemorates the Welsh winning the seven-year-long siege of Harlech Castle in the 1460s; against, of course, their neighbours the English!
Never mind though - it made for a great scene in a very good movie........
"Once you have flown, you will walk the earth with your eyes turned skywards.." - Leonardo da Vinci