Thank you so much for that info, JJ! I was considering driving up to Scotland and was about to check out car rental companies to view thier policy regarding transporting dogs (Tucker is a Golden Retriever). I must admit, I'm a tad wary as I haven't been back to the UK in 10yrs, so haven't driven on the left side of the road for quite a while. Don't know who I'd be more of a danger to - myself or others. Probably both!

JJ...sorry to be a pest..but could you tell me which Car Rental agency you're using from Manchester Airport? Thanks!

Poppycat, pest you are not, we are here to share experiences, and are glad to tell as we found it.
I wish was Manchester, but we are booked into Heathrow, which is miserable to get on the M25 at morning rush hour when most US flights arrive. However since pet clearance take 3 - 4 hrs we think we could get a break on traffic problems.
Our car is booked with Thrifty, decision was easy as wasn't much choice of national companies with a return location in our home town, Kings Lynn, Norfolk. Only Thrifty and Enterprise and after previous really poor service at Enterprise Heathrow I vowed I'd never rent them again. So Thrifty it is.
Even so, Thrifty prices seem quite competitive - and I booked through Thrifty.com and got as better price than through Thrifty.co.uk

I booked a intermediate size car just for 24 hrs one-way to be sure we have room for us and cases and the cat carrier. Then have booked a 2nd car, a cheaper compact for the ongoing week at a much cheaper price from and return to the same location in Kings Lynn.

One way is expensive , the oneway charges more than double the car price. I noted that if you have oneway and keep car for more than 24 hrs then the same high rate applies to every day you have the car. Can't remember who or where but something to watch for, just get multiple quotes with the one company to see how that works. But still rental oneway was cheaper and more flexible than an airport taxi.