Any question answered...

I need a new phone, I've already got a sim card.

Car phone warehouse sell the phone I would like for £100 sim-free or £70 (plus £10 top up) with a PAYG sim. I've heard the vast majority of phones from car phone warehouse are unlocked.

Any reason I shouldn't save myself £20 and get a phone with a sim and switch it out for mine?

Carphone warehouse phones are pretty much always unlocked, I have always just asked them in store and they have been happy to tell me whether it is locked or not.

They don't have phones associated with sims, they have big buckets of phones and big buckets of sims so just pull out one from each bucket when you go in.

This is unless it is some sort of special release or an iphone etc.