Yeah this is tricky.
Depending on where you are in the UK, it could possibly be cheaper to fly into Paris (not easy from Vegas where there aren't direct flights) and then drive/take the train over into the UK. It is cheaper to get pets into France than the UK, but it all depends on your calculations about a) getting the pets in one piece to France (i.e. no stop overs en route) b) distance/hassle to get from France to the UK. I think one or two people on here have gone that way. Unfortunately Vegas isn't the best origin airport for that :/
Also, you may need to watch it with Las Vegas because airlines do embargo out of desert cities either in certain months or when the temperature on the tarmac exceeds a certain amount. You should be good in March, but in case your timeline gets pushed back, you want to be aware of that.
Is there any way your mom can watch the pets while you are away? I know its three years, but it can be hard to find a place that will take pets in this country, and the vet and immigration hassles, especially if you are doing this from overseas, can be a big hurdle. We are currently still separated from our kitties and have been for a year now (with long visits to see them) because we aren't yet settled enough to have them come over, so staying separated isn't something I would really recommend unless you had to do it due to cost or complications. And I am not sure I would want to put them through a flight again either, the confused look on their faces when they came out the other end was both adorable and heartbreaking. If you have any way to not put the older cat through the journey, even though they are usually safe enough, I would avoid that.
Either way, it will be pretty expensive, but if you can pull it off in any way where you can fly them into a country other than the UK and enter via land, that may be the better way to go.
