You have impeccable taste even if you did change Sussex to Suffolk.
You're talking about my favorite beer.
I cannot help at all with extract brews. I made the switch to AG in the early 90's, and have never looked back, but I never had much luck brewing bitters from extract with steeping grains. If you can do it, then you are either a better brewer than I was, or today's ingredients are better. I suspect a bit of both.
If you want to go for a partial mash, or even all grain, then using a named English malt is essential. I use Maris Otter which is widely available in the US and is highly regarded.
For crystal, I use Muntons (50 - 60 L)
For a 5g brew, I use about 9 lbs M.O. and 1 lb crystal, which will come out a bit darker than yours.
If I am kegging (which is normally), I'll cut back on the M.O. by 1/2 to 1 lb, and replace it with an equivalent amount cara-pils. This seems to work well with kegged beer, but is quite overpowering if the beer is bottled.
I've tried small amounts of wheat, and various other specialty malts, but base malt and crystal (and cara-pils if kegging) gave a more authentic flavor.
I have added up to 1/2 lb brown sugar on occasions, and this has worked well.
For hops, I use Northern Brewer for bittering, Fuggles for flavoring, and EKG for aroma. EKG is typical for these bitters, but I prefer the flavor of Fuggles, and as Fuggles are also grown in Kent and Sussex, they should be OK.
Northern Brewer is not typical, but it's much cheaper for bittering.. I'll be going back to England later this year, and will see if I can get some English hops while I'm there.
For yeast, the Wyeast 1968 or White Labs WLP002 are both excellent (and noticeably better than any other liquid English style yeasts that I have tried).
For emergencies, Safale S-04 also produces very good results (but not quite as good as the liquid yeasts).
Hope this helps.
-a.
Edit>>
P.S.
To really appreciate these beers, you need to visit Southern England, and sample them in the pubs. The versions exported to the U.S. are poor imitations of the real thing.