i have an unlocked iphone and thinking it will be good to have a single sim i can use in all my destination countries: UK, Italy, Switzerland and Austria. If I wait till I get there, will I be able to buy such a sim, or will sims bought on site be country-specific ie work in italy but not in switzerland etc? What would you recommend?
A SIM bought in any EU country will work in all EU countries, plus Switzerland.
It will have a phone number from whichever country you buy it in.
thank you Chris!
I know a lot less about this than some others who post here, but make sure the SIM you buy will indeed work in all the countries you visit. The one I bought in Italy last year worked only there -- and was so described in the ads and in the store. That was fine with me, but I wouldn't have appreciated a surprise in another country. Ask the store clerk for this assurance, and also to install it and, if time allows, test it.
Also, keep in mind that a SIM purchased in Britain will come with prompts in English, while one bought elsewhere will prompt in another language. This makes a difference if, like me, you need the prompts to tell you what you're doing.
For what purpose will you mainly use your phone? If you plan to make local calls, then make sure the SIM you get supports that. If you mostly just want data so the phone works off WiFi for email, Facebook, maps, etc., then the calls probably don't matter.
EU roaming varies a lot by the specific SIM. Not all of them work in Switzerland (not part of the EU or EEA, so it is not required, but some companies support it there anyway). Not all of them work automatically - e.g. the GiffGaff SIM (very cheap in the UK) does not work for EU roaming unless it has been activated in the UK first and used there for a certain period of time (so I have been told).
I've had great luck with my Dutch Vodafone SIM. It does work in all of the countries you've listed. I bought mine on eBay (currently $11 USD shipped, comes with no credit on it). You can activate it in your home country and have it ready to go when you land in the first country. I've used mine on two trips so far and never actually used it in the Netherlands. It is easiest to create an account on the Vodafone NL website (has to be the Dutch site), activate the SIM with an incoming text message, and then add credit to the SIM on their website with your credit card, then activate a "You" bundle good for 30 days of data.
If the UK is your first destination, then you can probably pick up a UK SIM there and do the same thing if you get the right SIM. The UK Vodafone SIM is not exactly the same as the Dutch version, but it might work just as well for you.
More on UK SIM options here:
depending on what you intend to do, Google Fi might be the answer if the SIM is compatible with your phone. Calls are 20 cents/minute, data is charged at the same rate in the US or abroad ($10/GB), texts are included, since you buy the SIM here you know your phone number in advance, it is a monthly plan and you can cancel at any time. For us we make most of our calls to the US at the hotel using wifi (so free as per https://www.ricksteves.com/travel-tips/phones-tech/calling-over-internet ), the cell voice service is more for short calls.
I used giffgaff when we were in UK. They will send a SIM to your home in USA for free. You activate it here online and then buy a package, goodybags. Worked like a charm on my unlocked iPhone. I think they are on O2 network. My son was a student at Oxford and that's what the kids were using.