The best UK iphone plan DH and I have ever found has been with the network Three. They advertise that they're the only network that actually provides unlimited (they call it all-you-can-eat) data. Although other networks sometimes say they provide unlimited data, check the notes for a fair usage policy. Three's the only network we've found with no fair usage policy whatsoever. They used to say unlimited but have the fair usage policy, then they were up front and said it was a 1gb allowance, and now they've gone to actually providing unlimited data.
The data element was enough for us to go with them, but then the rest of the plan just seemed to provide soooo much more than other providers as well. It's called The One Plan. For an iphone 24 month contract at £35 per month (+ vat) it includes 2000 minutes, 5000 texts, unlimited data, free voicemail, and 5000 Three-to-Three minutes.
There's an offer on until the end of June where if you take out The One Plan the iphone 4 16gb costs £69 or the 32gb costs £159.
DH and I got them over the weekend and we're adoring them. His friends in London who have iphones are almost all on Three, so we know there's good coverage there. In our local area it's even better coverage than we had when we were with Orange. And since it's actually unlimited data, we've really been able to take advantage of everything. Most major free tv channels (like BBC, ITV, etc) stream their tv for free on their websites or through TV Catchup, so we've been able to watch that when travelling without worrying about hitting our data limit. We'll also never get close to using up our minutes and texts anyway, but will use even less of them because of the unlimited data. DH and I mostly text over whatsapp, or skype, or messenger, and all that data's unlimited. My sister from the US even skype video called with me on my phone for free! Seriously couldn't be happier with them.
Sorry I don't know about travelling out of the UK though...