Eastbourne locals.

friston does get bad in winter but there are a few groups thet ride there regularly.

the mud is less grinding paste than surrey hills. probably a bit more claggy in winter but less standing water. 

never ridden in pevensea. Done alot of flood risk work there though as its a floodplain! 

good steeep stuff near eastbourne. comedy gold riding it in winter.  

I've ridden on the levels, it's, err, level.  Very flat, a few hundred years ago it was tidal.  The paths there are mainly farm tracks, the surface is sometimes a bit muddy but a lot of the tracks have some surfacing and are ok.  There are paths that run through grass fields, sometimes with cattle.  These are hard going when the ground's soft and bone shaking when dry due to the hoof prints.

I'd take the gravel bike there in the summer, it's pretty peaceful but not challenging terrain.

I don't bother with Friston in the winter - its just a mud fest. I travel to Bedgebury which is rideable all year.

Downs can be fine but you need to know the sections to avoid. Its generally OK around the Firle to Alfriston bit and the area up around Beachy Head is OK too. Anywhere there are large sections of exposed chalk is a no go as it goes green and death will swiftly follow.

If you want gravelly stuff in Winter months, you can ride north from Eastbourne on the Cuckoo trail all the way up to Heathfield and link up with other routes in the Rother area. Have a look on Komoot for some inspiration.

https://www.komoot.com/smarttour/12859408?ref=wdd

I wouldn't bother with bridleways on the Levels unless we are at the end of a six month drought. Though they may be indicated as vehicular tracks they'll most likely be heavily rutted and muddy. Sort of thing you'll do once and can be pretty enough in a wild fenland type way.

There are plenty of short tracks and paths that will get you off the road, or far enough into rural roads that traffic is minimal.

When I got my first CX/gravel bike I just opened the OS map and tried everything. There's loads to keep you amused even though winter.

Bedgebury holds up well all winter. I've only just started going to Friston, which is more fun for sure, but bedgers definitely good for fitness maintenance. 

Friston is great, loads of trails to explore, but as above, winter brings a mix of mud and greased ice death on the chalk. Try riding the old school DH trail where it bus stops out of the bottom of X Files woods in December -eek!

Loads of cheeky stuff around the northwest of Eastbourne and as you head north, but bedgebury copes well in all weathers. 

There's also The Bull Track about 45 mins away if it's still going 

The mud does get bad but you'll learn which bits to avoid, or use some cycle paths or quiet roads to link up sections. Tops of the downs tends to stay mostly clear. After an overnight frost is particularly bad tho, turns the clay into a real sticky clag that clogs wheels in only a few metres.

I ride the Downs between Seaford and Eastbourne all year round, sometimes it's terribly muddy (especially after a thaw) but it's not normally too bad.  Overall I think it's a brilliant spot for sheer number of bridleways and endless route variations.

There is also a fair bit of gravel aka all weather track varying from groomed to rough double track with huge puddles. From the A259 nr Beachy Head through the golf course and down the cobbles to Jevington and then a hardpack climb followed by a couple of firm fields before another gravely double track down Windover hill to Alfriston and then back through the fire roads crisscrossing Friston would be a fault good loop. The byway just north of the Downs.  Cuckoo Trail north of Eastbourne. And the brilliant new A27 cycle path been Polegate and Lewes that also connects to lots of the quieter lanes north of the A27 if you go roadie.

Perhaps I'm used to it but the dreaded green chalk isn't everywhere, there are a couple of spots that require mincing down but you'll learn where they are.