"What's stopping an imdependent Scotland (or any other country) deciding to mirror the value of the UK pound? The only restriction I can see is potentially international copyright laws, but why then couldn't they keep parity but call it (say) the 'ellbie' or something."
There is nothing to stop an independent Scotland (or anywhere else) from simply using the pound without an agreement, though it is generally considered a poor choice, given that its a complete surrender of monetary control.
As far as having a separate currency with its value pegged to the pound, imagine the following scenario: many Scots with savings worry about the peg being maintained, and therefore decide they don't want their life savings in the new currency, and so change them for sterling, dollars, gold, etc. This floods the market with the new currency, and naturally that forces the price (ie the value) down. This forces the Scottish central bank to step in to support (ie buy) the new currency with its reserves to maintain the peg. But these reserves are limited, which everyone of course knows, causing more people and businesses to try to get their money out before its too late. At some point the Scottish central bank has to stop buying, or simply run out of reserves. The currency crashes and the peg is abandoned. And this doesn't even consider speculators who anticipate this exact possibility, place their bets on it, and then work to make it come about - as George Soros forced sterling itself out of a similar arrangement (the ERM) in the nineties, making himself billions.
Currency pegs used to be a lot more sustainable, since capital controls could physically prevent people from changing their money, but that isn't how the western world works these days. If Scotland were to launch its own currency it would pretty much have to be free floating - which seems like it would be the logical choice anyway, given how much of the SNP's rhetoric is about escaping the control of Westminster, which is surely only possible with monetary independence. I guess their analysis is that asking Scots to vote to have their savings redenominated is a non-starter, hence their absolute insistence that the UK secretly intends to offer a full currency union in the event of independence.