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For me, personally, things like the Tour of the Cornfields (different routes different years, can be borrowed off strava) or the Bicycle Ambulance byway bash (only been done once to my knowledge but again, have a google and you should turn it up) routes are quite nice, also the bridleways around the roman road, the icknield way/peddars way.
Mostly I just open Komoot, switch to open cycle map and look for the green lines, try to stitch as many of them together as possible, route to a cake shop...done. It's often worth following the London Liverpool street train line and seeing if you can hitch a ride to the south and then ride back with the prevailing south westerly wind. If you're on komoot dm me and I'll send you my profile which isn't amazing or anything but might give you some ideas (or ideas of people to follow)
Not saying it's like, world class riding or something (I've not ridden much of the world) but I would say it certainly could justify having a gravel type bike (with or without mudguards).
Decent amount of hardpack dirt around cambridgeshire too IMO. I used to be of the "fnar there's no grav in the uk" but I've changed my mind lately. I think the main difference is that whilst there's plenty of bridleway, there's not huge amounts of linked up bridleway, so you end up on the road not just cycling to and from, but also during.
Also in winter the hardpack dirt turns into cloying, destructive mud which fills my mudguards instantly and in the summer it sets into rockhard washboard with 0 moisture for my mudguards to catch, but I still have mudguards on my bikes whenever possible. It's the principle of the thing.