Personally I wouldn't go crazy on things like roast barley, which is getting away from an English porter. A classic English porter of the mid-19th century would be
82-87% Chevallier pale malt
10-15% brown malt
3% black malt
1.055-1.060 OG
and that's all for the grist. Chevallier is a historic variety that is available in small quantities from Crisp, but you could substitute it with something like Maris Otter pale with a bit of crystal.
The benchmark for modern porter is Fuller's London Porter, which is amazing on cask but which translates better than most to bottle (and has reasonable distribution which should only improve after Asahi bought them recently). Typical clone recipes are along the lines of :
76% UK pale malt
12% brown malt
10% crystal
2% chocolate
1.053 OG
Historically porters could be hopped to 50-60 IBU and left to age with some Brettanomyces and whatever else was hanging around, modern ones would typically see 35IBU added at 60 or 90 minutes, and 0.5g/l of hops at 10 or 15 minutes. Fuggles would be the classic hop to use but any British hop would do, it's not that critical - or use Willamette or Styrian/Savinjski goldings if they're what you have locally.
They're full-bodied beers, so you want to mash fairly high and use the most characterful, low-attenuation yeast you can find - this is not a beer for Nottingham.
I'm not sure how pilsner would work, other than it would end up thinner and less flavourful than using European pale malt, let along a British pale malt (which gets cooked a bit more than the Continental equivalent, and is made from a more characterful grain to start with).
This thread shows you the difference between Weyermann pilsner and their pale malt, a British pale malt would be a bit darker still. It's maybe not something to worry about for your first batch, pilsner will get you pretty close, I'd just pay particular attention to the mash and attenuation as above. One option would be to add maybe 3% Weyermann Carapils, another would be to cook it a little in the oven, I'm sure someone's worked out some timings somewhere.
According to the usual (not entirely reliable) charts, Carafa (Special) II is the closest equivalent to chocolate, and Caramunich III the closest to crystal 60.
Brown is the tough one that doesn't really have an equivalent - if you're in France then the charts suggest MFB Kiln Amber is close. Durden Park suggest the following schedule for making your own from UK pale malt :
30 minutes at 250 degrees Farenheit
30 minutes at 300 degrees
30 minutes at 350 degrees, stirring every 10 minutes to prevent scorching
Do try and find a bottle of the Fuller's London Porter if you want to get a good idea of what it's about. I guess the Meantime porter is the other one you're likely to see, I've not had it in a while but memory says it's not as good.