In August look out for deals on HP 840 G8, HP 845 G8, HP 840 G9, HP 845 G9.
For something more portable but with a smaller screen to work on look for HP 830 G8, HP 835 G8, and the G9's.
Dell and Lenovo business laptops used to be worth buying, but they've moved to soldered RAM, and the recent Dell premium business laptops seem to be less well engineered than they once were, resulting in me no longer being able to recommend them.
When shopping for laptops enter the model you're interested in in the ebay search bar and order them by lowest price +P&P first.
Then work your way through them till you come to a fully working one with no faults on the screen and a chassis and lid and baseplate that's not too bashed about. A few dings and dents are OK, but a cracks or tears in the fabric of the laptop are less good. I also look at the condition of the keyboard and trackpad - any signs of wear from a lot of use in those 2 areas would reduce how much I'd be willing to pay for it compared to one with a hardly touched looking keyboard and trackpad.
When comparing laptop deals I bear in mind the price I can get RAM sticks, SSD's, batteries, chargers. All of which are relatively inexpensive and easy to fit. So that, for example if there's 2 equally good same model laptops for sale, one with 8 GB, the other with 16 GB, I'd buy the 16 GB if the price difference was less than £15. If the price difference was more than £20 I'd buy the 8 GB and as soon as I got the laptop I'd check what exact RAM stick it has and buy another.