That’ll be the Cuilfail Tunnel… about 200m from where I live.
If you go past there there’s a nice clear sweeping road (A27) towards Seaford… or, if you go through it, there’s the A26 to Tunbridge Wells and beyond which takes you back up towards London and is quite nice. A21 is dull but there are lots of good B roads about… my local stomping ground, so to speak.
Thanks Hanna, I rode that road donkeys years ago and couldn’t remember where it was. A mate and I rode through it a dozen times just for the sound bouncing off the walls I hope this helps the OP.