Thamesmead, Greenslade, Erith, Crayford, Dartford, Gravesend, Strood, Chatham, Gillingham, (I'll let Rochester off the hook as it has so much beautiful Georgian architecture, although it's still a bit of a dump overall.)
and their mirror image towns on the other side:-
Dagenham, Purfleet, Thurrock, Grays, Stanford le Hope, Tilbury, Basildon and Canvey Island.
All quite jaw-droppingly grim.
(almost all of the rest of Essex is really far, far lovelier than most people realise though - honestly! Especially the north of the county, it's beautiful, apart from Braintree, but that has some lovely villages around it)
Coalville.
Luton and Northampton.
Wolverhampton
Reading (apart from the nice bit of Earley)
Bracknell, Newbury, Wokingham, Slough
Croydon
Dover/Margate/Ramsgate/Hastings (although there are efforts afoot to gentrify Margate and Hastings, a la Whitstable. They've got their work cut out, but you know these pioneering arty types.
)
Huge swathes of what used to be Middlesex - Feltham, West Drayton, all the areas around Heathrow (except Datchett/Wraysbury) and Hounslow. (and all you people who live in Hounslow stop pretending it's Twickenham.)
90% of Lancashire. There are too many godawful, grim, depressing towns to even have the energy to type them. Countryside stunning, people mostly friendly and warm, if a little erm...over-exhuberant and a bit too fond of the pub, but Good Lord, its towns are depressing, insular places. I have been going there pretty regularly for most of my life, all over Lancs, and I struggle to name a nice bit, so I'm not tarring a whole county with the same brush as Burnley, before anyone says that. (but Burnley has a special place in my heart for being super-awful.)
And everyone says 'Ooh Clitheroe is lovely, and Hebden Bridge is lovely (ok that's technically Yorks, but it's the nearest trip out of hell if you are in the arse end of Lancs) but guess what? They are not even that lovely. they are just a lot less grim than everywhere else. It's all relative.