Test Book or any other diagnostics will not show up worn or damaged mechanical parts,
e.g. air springs, wheel bearings, piston rings, bearings, oil pumps, tyres etc. etc. as it is purely electronic and reliant on sensor, switch inputs and outputs.
Not all cars are as robust as others especially with a relatively high mileage and perhaps a dubious history. That said, and what has been posted on the forum, well maintained cars have had major problems..slipped liners is a classic example, on engines that have done fewer miles than yours. It's easy to say "Mine has done 190K and is perfect", that may be so, but as everyone knows that does not apply to others. My Disco V8 had less than 90K on the clock, full history and the camshaft had gone, noticed only when I came to change what I thought was noisy tappets.
Think about it..the lobes are worn, the valves do not open fully and for a shorter time therefore less volume of air/petrol mix gets into the cylinders. The lambdas sense that the exhaust gas is weak..it calls for more petrol.
I hope that the lambdas work. I know it's one thing after another and disheartening but for want of a valley gasket & seals it would be well worth a few hours work to pull the inlet manifold off and have a look at the state of the camshaft just for peace of mind. I am sure that RPI would show you what a new one looks like in order that you can get an idea of what you are looking for. A new camshaft and tappets including chain & sprockets is available for around £150 as a guide.
See how you go. If you need more info. PM me and I will give you a list of options regarding prices and shopping list.