Try telling small businesses in the UK that transaction fees are merely a rounding error when it would probably be fairly massive to them if they're already struggling to get by. It might not be massive figures wise, but a lot of these businesses would in all likelihood be affected by it.
Of course these companies would be affected. But harsh as it sounds, this is cherry-picking. The impact will be felt over the whole of the UK, not just those who deal between the two countries. It makes no sense just to consider a few of the aspects.
I can't say I know too much about the bonds stuff. Obviously I don't claim to be a financial expert at all, I'd only claim to know the basics, but I've not really heard this issue of bonds being brought up a lot so far in the whole thing. Even though it would perhaps come into play, a large part of the whole idea of the currency union is that not a lot would change, so I'm not sure why additional bonds would suddenly have to become involved if we're sharing a currency.
OK, I'll simplify things for you. The UK - and pretty much every country, barring the likes of North Korea (maybe?) - borrows money. A lot of money. The UK raises money by issuing bonds - kind of like an IOU - upwards of £100b (I think) every year. People like traders, funds and other countries buy these bonds. The UK repays these slowly, with interest.
Of course, each country is different. The UK is very likely to repay, but a country like Zimbabwe is unlikely to do so. Consequently, the interest rate charged on UK bonds is lower than Zimbabwe's, to factor in stuff like defaulting, sanctions, etc. (because investors will want higher returns to offset the fact that the money might not arrive from Zimbabwe in the end). The UK is likely to repay due to the strength of its economy and history of repaying debt. Zimbabwe... Less so. This is another word for "risk" in many ways.
So what happens under a currency union? The UK will find itself suddenly not in control of a not-insignificant part of its economy that is backed by the pound. The currency union agreement will likely say something along the lines of "monetary policy can change only if the Bank of England and (hypothetical) Central Bank of Scotland agree". So what happens if the two disagree (not outlandish, given they are now competing countries)? The UK will find itself hamstrung by this, so it has less control over monetary policy. Scotland might also decide to do something economically stupid (maybe only with hindsight), which will hurt the pound.
(I'd note that a lot of this can apply in reverse, as a negative to Scotland, the difference being that as the bigger future nation, England is better-equipped to recover.)
All of this adds up to higher "risk" for the UK. What happens when risks increase? So does the interest rate on bonds. The UK then finds itself effectively being able to borrow less from the markets. This has a knock-on effect on the rest of the country - the government can't spend as much, so needs to cut costs; international companies based in the UK also take similar hits...
Scotland can concede this and maybe offer more control, but that is less "independence" and more "vassal state". And politically impossible in Scotland.
You say that the UK don't want their currency to be controlled by an independent country, but that could happen anyway if we went independent in that Scotland would merely say that they're as entitled to the pound as the rest of the UK are, and could simply attempt to use it without the rUK's consent. If that were to happen, wouldn't it make more sense for rUK to negotiate a currency union instead of us simply using it, which would only cause further problems?
True, nothing is stopping you from using the pound without permission. You can use the US Dollar, the Japanese Yen, or even Bitcoin if you so desire. However, you will be at the mercy of UK monetary policy. If the UK one day decides to print more money, Scotland will take the inflation hit, even if Scotland is comparatively OK. Scotland wouldn't be allowed to print pounds (in the UK, that would be a criminal offence and the notes considered counterfeits... If another country did that, it could be considered an act of war.
Nazi Germany tried to do this with the pound), so it couldn't stimulate its economy if it needed to (i.e. Greece). If Scottish banks fell into trouble, the Bank of England would not lend as a lender of last resort - so it would more easily go bust (although I think that practically, the Bank of England would lend
some money due to systemic risk and the fact that the two countries still share close ties).
And, of course, it wouldn't be considered "independence".
I know Darling has said a lot against the union: the point was merely to explain that their are major Better Together people out there who have admitted that it's logical and desirable. Preferably I'd rather Scotland could run it's own affairs without any interference in monetary issues, but if it works out to be the best option for both countries for now then I'm happy to secede some national sovereignty in order to ensure a stable currency that works for both countries.
The thing is, I don't think it will be "some" sovereignty. I think it will be a lot. At worst, it will be in proportion to the sizes of the economy.
I think the most sensible thing would be for Scotland to setup its own central bank and its own currency, pegged to the pound (maybe with some range) - maybe even 1-1. Over time, the pound sterling would be phased out, and the peg removed. The central bank will let you apply to the EU as one of the criteria, and Scotland would be able to set its own monetary policy. It would then be fully independent as can be. Scotland will inevitably need something like a central bank anyway, should it become independent, so you may as well get started on planning. Of course, it's not easy setting up a central bank... Which is presumably one of the reasons why the White Paper doesn't mention it as their first choice.
There are already Scottish pounds in circulation, albeit not of legal tender in the rest of the UK, so some mechanism already exists. This isn't really the same as a new currency, though, as Scotland has to pay the Bank of England an equivalent amount in pound sterling (or something like that). They're just changing the pictures on the notes in an environmentally-unfriendly way.