I read this UK paperback about six years ago but it could have been older than that. Can't recall anything about the cover.
The story began in a post-collapse society (update edit: I think it was a general crash of society worldwide, caused by over population and breakdown of control systems) now ruled by city-states, still with advanced tech. They all had their own surroundings they could protect, with 'badlands' in between.
A roving war party from one city stumbles across an agricultural village way out in the badlands that had been set up by another city, so they charge in and take over for loot and rape and pillage.
Held captive permanently with the war band is an immortal Englishman whose inner body is some blue goo stuff. When injured he heals very quickly provided more blue goo is available, this can be obtained from tyres from high tech abandoned cars.
This man apparently had a (unwitnessed) close encounter back in our time near Manchester UK and his body was changed by the aliens. After living through decades of post collapse misery, he is now in the former USA and is the local warlord's plaything and often finds himself being tortured for entertainment, because he heals quickly but still feels full pain.
The war party put implants into the captured villagers to control them. There were gory descriptions of men punching each other to death and women screaming and running and getting gang raped, however the implants only let them run slowly in circles so they are easily recaptured.
(I think) airships eventually arrive to rescue these villagers and a battle begins.
A further snippet of this book has just come to me; there's a derelict mansion/house in this badlands village, and the war party use it as their base. I want to say they have a female "combat advisor" who (before they approach this manor house) inputs details of the buildings layout into their phones - showing that some tech survives.
The blue goo man is wandering around inside it and trying to think of a way to escape. He lifts a rusty tin box off a shelf and looks inside; he is mega delighted to find it's crammed with tea.
The outer leaves are mouldy but there's enough good stuff in the centre to make several pots of tea. Being a proper English bloke he immediately brews up, because he hasn't had a cuppa in fifty or so years.
I want to say he somehow ends up on another planet (but this might be a different story) that is full of people who have had this "alien blue goo treatment." They are made to reenact different battles every day and then it resets (like Groundhog Day) and I think he was always in a WW1 trench fight.
After thousands of days, from initially getting killed very rapidly, he'd become super expert at eliminating the threats around him so he could laze around and chill all day.
I think he eventually manages to contact and communicate with the aliens and negotiates a way out.
He gets very badly tortured including demasculating and amputation to get some old tech info.
This girl who has a crush on him runs off to an old garage and brings back some blue goo tyres. She cuts them into small chunks and packs them around his wounds.
He is somewhat bemused, the next day when he has recovered, to notice she has ensured he now has a much larger manhood, and she then starts to openly flirt with him.
A further memory I've just had... He meets a fellow 'blue goo' human who's almost insane, this person had somehow been on a spaceship that was caught by the Sun's gravity and crashed into it, he spent years in agony there as he couldn't die. He finally, after many tries, succeeded in getting himself on an ejecta and was shot out into space in the right direction. He then endured years of suffocation until he kicked loose in the direction of Earth and was eventually caught by gravity. Then a screaming re-entry and slammed into the ground, some days later he feebly crawled out but now cringes on hot sunny days.
A further memory! Very near the start of the book a female paratrooper (possibly the same "combat supervisor" who puts maps onto people's phones) and her squad are dropping from a dirigible on a raid of some sort. She hears a terrified scream as the blue goo man goes hurtling past her, I think the crew simply threw him out without a chute
Update October 3rd 2020
There is a recent possibility that it might be 'Something More' by Paul Cornell. However the reviews/blurbs I can find online are (so far) failing to convince me