Not sure if this will have been posted or not.
I'm going to New York in February and I have been a bit concerned about not being able to use my phone (particularly for maps) whilst there for fear of racking up a ridiculous bill.
As you may already be aware three offer "Feel At Home" which allows you to use your allowance whilst abroad at 16 locations (listed below). Now, for Pay Monthly customers, you have to have been with three for a minimum of a month. But if, like me, you just want the usage while you're away without switching providers you can just get a pay as you go sim. If you add an "add on" you can use the allowance from it immediately whilst abroad.
For the £15 add on you get:
Unlimited Data (Capped at 25GB whilst abroad)
This lasts for 30 days (so should cover most holiday trips), which seems very good value. Additionally receiving calls and texts whilst in the listed countries from the UK won't cost you anything (unlike many other providers).
I can't think of a better way of doing it really. I'm sure I'll be getting lost frequently, so want to be able to use my maps whenever I like, plus this way I can also make the odd call home to check in with things back there.
The countries this will work for are:
Republic of Ireland
EDIT: To add the add-on simply do it at sign up stage if ordering the (free) sim online. Alternatively, top up £15 on to an existing three PAYG sim and then call three to get them to convert it to the £15 add on.
EDIT 2: Added new countries i.e. Spain, New Zealand and Canary Islands