I got this news - that parts of Britain are using the Euro - via a Conservative acquaintance who linked it on Facebook with the comment, "no no NO!".  In other words, Conservatives are very uncomfortable with the idea that people might of their own free will use a Europe-issued currency.  Their nationalistic-authoritarian instincts are continuously at war with a superficial belief in economic liberalism, the view that people should in general be trusted to take their own decisions.  They share a lot with centralising socialists in this regard - who don't like losing the ability to run amok, fiscally, without external constraint.

I am not proposing that we immediately 'ditch the pound'.  But allowing people the freedom to use whatever instrument they like in economic exchanges is one supported strongly by Hayek, the favourite philosopher of Margaret Thatcher and one who wrote "Choice in Currency: A Way to Stop Inflation" in 1976, which said:

BUT WHY should we not let people choose freely what money
they want to use? By 'people' I mean the individuals who ought to
have the right to decide whether they want to buy or sell for
francs, pounds, dollars, D-marks, or ounces of gold. I have no
objection to governments issuing money, but I believe their claim
to a monopoly, or their power to limit the kinds of money in
which contracts may be concluded within their territory, or to
determine the rates at which monies can be exchanged, to be
wholly harmful.

 It is, obviously,not the model the EU authorities chose to adopt, but one sufficiently interesting to be taken seriously by Otmar Issing in 1999, who somewhat hopefully expects the IEA to applaud the Euro on this basis:

Therefore, although the path taken to achieve denationalisation of money has been very different than that advocated by Hayek, the ultimate objective being sought by Hayek, i.e., monetary independence from political interference and price stability, have, to all intents and purposes, already been achieved.

The "Hayek would have approved" argument. 

As is often the case, Samuel Brittan has already covered this, a while back in 2002, writing:

Instead of just saying yes or no to Euro membership, the British government has the option of leaving it to private citizens and traders to use the euro if they wish. Some, but not all, of those who welcome the idea hope that it will lead to increasing use of the euro in the UK. British exporters and financiers were perfectly free to make contracts in euros from the time the currency was launched in 1999; and, with the advent of euro notes and coins in 2002, visitors and returning tourists have been able to use them for payment to whomever will accept them.