Interested in peoples preferences for where they buy currency in the UK before travelling. I use bestforeignexchange (may be London only) which give great rates, but I am really fed up of them palming me off with huge denomination notes, even when I ask specifically not to have them. Has anyone been anywhere where they will actually give you the denominations you ask for (ie treat you as though you are the customer and give you what you want) and not make excuses saying they don't have enough of x notes etc?
I use Travelex. I also tend to receive large notes. You might be able to ask for smaller demoninations if the currency is a widely used one such as US dollars. Failing that, I ask hotel reception to break down the notes for me
It's usually better to use overseas ATMs, to withdraw your cash as you need it, rather than to use any currency exchange facility in the UK. (The better exchange rate you'll get normally outweighs any fees that you might have to pay). It's also more secure.
However some of the best rates on the High Street are available from 'the usual candidates' but by ordering online in advance. For example, the Post Office usually has reasonably good rates anyway for 'walk-in' customers but those who've pre-ordered their foreign currency online get a much better deal (with even better rates if you're buying larger amounts of currency):
http://www.postoffice.co.uk/foreign-currency
As far as ATMs are concerned - it depends which card you use and whether it has hidden loading charges. I have only used ATMs as a last resort and the last time I did that was about 9 years ago. There are better options.
If you know how much money you are going to spend then maybe it's better to get money in advance
We never do so we use ATMs: convenient and secure. I hate having to travel with a large pot of cash which can only shrink :-)
I used Marks & Spencer for chinese currency. some countries pretend to have ATMs but don't - like Algeria. Well, they have them, but they don't work. Some countries, however, use currencies which simply aren't available outside their borders. You just have to use the exchanges at the airports when you arrive. I think that included Libya, but I imagine you aren't going there.
Use Travelex facility where you pre-order on-line (up to 4 hours before pick up time) and collect at airport of your choice. Better rates than buying it directly at the airport, and when they tried to give me lots of big note dollars last time I asked them to change it (so I had single dollars for tipping etc) and they did without hesitation.