I'm outside Edinburgh in West Lothian and we moved here in May this year and rented for six months. We had to pay an application fee (which was £150 or so if I can remember), a security deposit of one month's rent, and the first months rent. Our place didn't allow pets though, and with three of them, I imagine it may be somewhat difficult and more expensive.
The six months in advance isn't normal for anyone living in the UK. But if you have just moved here, don't have a credit history here (remember, your US credit means NOTHING in the UK), and don't have several months bank statements, references, etc., renters are more likely to ask for this assurance of six months rent from you.
Any chance you can leave your kitties behind for a few months, move here and get jobs and get established from a B&B or something and them bring them over? Might be one option for you!