When I've shopped price Hertz usually comes out about as good as any. Avis tends to be a bit more and Europcar quit a bit more. Sixt is usually similar and occasionally cheaper than Hertz, especially when I fly into Manchester. We also did the Arnold Clarke one for a rental in Inverness. On that one occasion, probably because it was in Inverness City rather than an airport, Clarke was the best choice.
I think I've always done Hertz at Gatwick, Sixt at Manchester plus that one Arnold Clarke rental in Inverness. In each case that was the best deal I could find.