I think flowerista addressed the 3G issue. I don't have 3G on mine because I don't want a contract, but my understanding is that you'll need a contract with SIM (the thingy) to activate the 3G (thingy). I have no idea who provides the best service, or if perhaps only one network is authorised by Apple.
Sorry, I'm rather thinking of it as a phone. You can get contract phones which come with SIM. (The SIM is the wee piece of plastic and gold that sat under the battery in my ancient PAYG phones; it contains the phone number and will store some of your contact numbers)
You can get PAYG phones with a SIM, linked to a particular network, but if you want the handset and not the network, you can change the SIM in a PAYG. But you have to be able to "unlock" the phone; for some reason, this service is often provided by corner stores and all-night newsagents.
I'm just thinking that an American iPad might come with an American SIM, locking you into a network which isn't actually available here.
If you buy an ipad from an Apple store it will not be sim locked. At least not the ones in the UK but your friend can confirm that before he buys it.
When you get it, pop into your nearest T-Mobile store and ask them for a free Microsim for iphone. Once you put £10 credit on the SIM, T-mobile should activate 12 months of Internet on it for free. Put this into your ipad and you should be ready to rock.