Great to hear you've settled on the RX.
Yes, the camera and big mirrors do help when manoeuvring, and as you know from your Land Rover being that bit higher up gives better visibility anyway. I think RX is a bit more trim than some other cars of this type - I'm currently working on a project which includes a new car park and I've had to allow for an area with 50cm bigger spaces for the directors/visitors in their Range Rovers, X5s etc.....
My search on AutoTrader was pretty similar to yours, using that it's interesting to see how many are white - 33 of six other colours versus 18 in white alone.
Many of us on here have travelled to buy the right car, it really depends how quickly you need/want it. I would much preferred to have bought it from the helpful, friendly chap at my local dealer but even within their group they just didn't have the right car for me. Four months later they finally do, but I couldn't have waited that long. I believe the warranty covers the car at any dealer, so even if you have a problem you don't need to go back to the people who sold it to you.
I did have to go 100+ miles back to my dealer, but that's because of an issue that wasn't covered by the warranty so I had to exchange my first RX for a better example.
What sort of mileage will you be doing in it and how long do you intend to keep it? Just wondering if you want to find something with low mileage, or whether you can accept higher mileage but with good history - the latter will save you a bit. Sticking to Lexus dealers, the nearest I can find to you that meets your criteria is a black F-Sport in Sheffield with 63k at £21,450. Lower mileage examples are usually £25k and upwards.
If you could make it down to the area south of London (Kent, Surrey, Croydon etc) then you'd have a good range to choose from various dealers, perhaps make a weekend of it? Shame the silver Advance at Glasgow has a tan interior, but could still be worth a look at? Having done just under 50k you'd have an idea of how well that colour takes everyday use ad they seem to be offering a good deal with various extras thrown into the package.