I also highly recommend Arnold Clark. I've used them on my last two trips to the UK, and will continue to do so whenever I can.
Earlier this year I rented directly with Arnold Clark (17 day rental) and then in 2023, I rented from them through their broker, Celtic Legend (14 day rental) for my trip to Scotland. Both rental experiences went very well. I would suggest getting quotes from both Arnold Clark directly and Celtic Legend, and see which is more favorable.
Either way, you will be dealing with Arnold Clark once you rent the car and they are a pleasure to deal with. Their agents explain everything to you and make sure that you understand the fine print. They are good about upgrading (they once upgraded me to a Mercedes from an economy), and extremely courteous and helpful. I will always use them in the UK for future rentals.
https://www.arnoldclarkrental.com/
https://www.celticlegend.co.uk/car-rental/
In terms of choosing a car, look for something that is big enough to hold 4 adults and your luggage. There is a recent thread about car size in Scotland. I will try to find it and post it here.