Good indication of prices here: http://www.amazon.com/s/ref=nb_sb_ss_i_0_6?url=search-alias%3Daps&field-keywords=ipad+2&sprefix=ipad+2

Also look at BestBuy.com

Will you need a mains converter to use it over here?

Not checked but most computer items are universal voltage and will work anywhere. However, you will need an adapter to convert the US flat 2 pin mains plug to a UK 3 pin. Cost about £5.00.

I would check the likes of Walmart, KMart or Target in addition to (or perhaps instead of) Best Buy. Best Buy aren't particularly cheap by U.S. standards, although presumably busdriver mentioned them because are an electronics store. I imagine that Best Buy would likely still be quite a bit cheaper than the U.K. and would do in the unlikely event of not being able to locate a Sprawlmart within a two mile radius.

If you have a TK.Maxx near you,keep an eye open in there, they have been selling IPads as cheap as £299 in the UK

TC