First and foremost it depends how much money you're exchanging.
The post office states that it does not charge commission fees. As do most high street banks. Some have policies that you have to be a customer, some do not
Marks and Spencer also have money exchanges. Online now I have noticed that Sainsbury's also have a bank and so do money exchange, but I'm not sure of their specifics, since they're in competition with the above its likely there very similar. I would presume tesco have one too!
Check the rates. Usually there not much difference and it's whatever most convenient. For me it was using the bank.
Everyone has to make money from it - you will get charged commission OR get a lower exchange rate. Take your pick! Banks are best, and Marks and Spencer aren't bad either, though your nearest M&S that does this is in Enfield. The one place NOT to use is anywhere that calls itself a Bureau de Change. I see these in central London with queues outside and I want to scream at the queue that this is the most expensive way to change money! Commission-free, yes, because the exchange rate is awful.
The obvious answer is to try your bank. Order in advance as they aren't going to have large quantities of foreign cash hanging around just in case anyone wants some, but a London branch will have no trouble getting some euros you can have next day. Mine lets me order online, they make up a pack of money and I can collect it next day. If you do that, take your passport as ID when you collect it. Your friends clearly have no idea of banking services - this is BASIC to banks. I know they don't obviously display that they do it but ANY bank branch can.
Travelex at the airport actually give a reasonable exchange rate, but anyway, if you can get it done before you go and have one less thing to do at the airport, that can only be good.
Another thing banks will do is sell you travellers' cheques. I know everyone will tell me this is old-fashioned, but cash is obviously vulnerable to theft and using your debit or credit card in Greece (which you can do) is going to be expensive in conversion charges and fees. And no doubt you will come across little cafes and tavernas that can't cope with cards.
Travellers' cheques are theft-proof if you follow the instructions carefully, sign every single one before you go and keep a separate note of the cheque numbers. If they go missing, you can call the bank and get replacements. Then when you want more euros, take them to any Greek bank with your passport for ID and cash them, only signing them again in front of the cashier so they can see it's you signing. Both signatures on each cheque being the same is one of the security checks.
When you get home, your bank will happily change anything left back into pounds and even pay it into your account. It won't take foreign coins, though, so try and use them up before you come back. This is why airports often have charity boxes that will accept any currency. It's as good a way as any of getting rid of your last bits of small change. You can't change a small amount of coins, but the charity can collect bags of them and find a bank that will take them.
Almost everywhere charges commission. Those places that don't make up their money by offer a crap rate of exchange - avoid the Post Office for this reason. The first place to look is your own bank, as a customer you may get reasonable commission or a more competitive rate. You really need to compare online, but rates vary daily.
People often prefer to change a small amount before they go, say ￡30-50, so they have enough for bus or taxi to where they are staying, then change money in the country after arrival, as some give a better rate.
Post office
When you get to Greece you can convert easily to Euros inside the airport. Yes, they charge commission everywhere, but I've foun the airport is about the cheapest.
Have a safe trip :)
Go to the Post Office - they do not charge commission and they offer competitive rates - this is the easiest and most straightforward way to change you cash. Do not do it in the airport as the commission rates they charge are ridiculous
Use ATM to get money in Greece- just check with your bank about any fees
The post office or M&S money are generally the cheapest options.
