Problem with Three is coverage, tends to be a bit all or nothing, mostly nothing around here, despite me being slap bang on the M25.
Personally Ive always been a Voda or O2 person due to their Network Coverage in General, having said that EE are making some big waves now mainly because they still heavily rely on the Sister Co's Orange and T Mobile as a Network Backup, but in essence have a 3G/4g Network all 3 in one.
My advice is all about doing your homework before you commit, almost all the Networks give away PAYG Sims online, so a bit of a mess around to see which one would siut etc, sure you get the idea.
And yes, PAYG can end up more expensive than Contract (Sometimes Cheaper) but if you looking at chunking loads of Data about ie, Skype or Simular, then have a look at some T Mobiles PAYG Bolt On deals, 30 Days Internet can be had for Fiver a month. .
Also, do you really need Skype, if your looking at Voice to Voice Call Overseas have a look at Rebtel, been using them for years, ok, its VOIP but they set the Calls Up using simple DDI Numbers meaning each person is using a London (0203), 0800 or 03 Numbers in the UK which come out of your Mins Pack anyway, and the same deal for Callers to the UK in other Counties (Local or Free Reate)..