Ask Shelly: Car seat for three days in Scotland?

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I am traveling to the U.K. and France with my 3 year old. For a teeny portion of our 3 weeks trip to Europe (for 3 days only), we will be renting a car to drive from Edinburgh to the Highlands. I really do not want to lug his car seat on multiple airline and train rides when we only need it for 3 days! We have a CARES harness for the plane – which is much more comfortable for him that being s...